Introduction You may wonder what subnetting is and its purpose. A loose understanding is the following.Subnetting is a process of partitioning a network into smaller (sub) independent network. The smaller network is called subnet. Subnetting relates to a good network design. One aspect of a good network design is to optimize the IP addresses that you have. Don't let IP addresses unused or go wasted. This is true especially when you have to pay to have those IP addresses, or you have limited IP address range to work with. The example of pay IP addresses is Public IP addresses that you retrieved from your ISP. You have to pay certain amount to have static IP addresses dedicated to you from your ISP. When you ask more IP from them, you have to pay more. This FAQ of subnetting discussion will be only in a perspective of IP network version 4. Further, the discussion is trying to build a basis of understanding on how subnetting functionality plays fundamental part in IP network, especially in relation and/or application to IP routing and switching as part of communication ways in IP network, and to best-practice network design in general. Hopefully after reading this FAQ, readers can have better understanding and better appreciation of subnetting beyond a mere mathematics or beyond a certification test passing. As with any learning process of new knowledge, there is no "cheating", meaning you need to understand the basics. There is some mention about binary number system (which you will find out soon) in regards of subnetting learning process. However unlike any other source, this FAQ tries to minimize the use of binary number system and tries to maximize the use of decimal number system which is more common among humans. The objective of this FAQ is to try to introduce you subnetting knowledge with less math to avoid dry reading and more on the "soul" of subnetting itself to embrace the essence. Binary Number System (2-based Number System) The most difficult part of understanding subnetting is probably the math (the calculation). As you can see from the links above, subnetting involves binary numbers. Yes, you are required to understand at least the basic of binary number system in order to understand subnetting process. Binary number system is used by any computers based on their nature of "on" and "off" state. Unfortunately we humans are used to decimal number system, hence create a gap. This gap leads to some kind of confusion to those who are just learning networking and subnetting. But no worries! There is an easier way to understand subnetting with less theory and more practical approach. The key is to keep using decimal number system with binary number system in mind. Before we begin, you need to refresh your math on power. Following is an illustration. 2^0 = 1 2^1 = 2 2^2 2^3 2^4 2^5 2^6 2^7 2^8 = = = = = = = 2x2=4 2x2x2=8 2 x 2 x 2 x 2 = 16 32 64 128 256 Binary system number is based on power of two (2^n). This number system shows that you can only double the current number to have the next bigger number. This number system also shows that you can only halve the current number to have the previous number. The Binary system number hence introduces the concept of half-and-double size. To explore further, check out the following table. On the table, note that the next bigger number is always double the size of the current number. From different approach, the previous number is always half size of the current number. As you may see, there is no other way to have the next bigger number of the current number but to double size of current number. Similarly, there is no other way to have one smaller number of the current number but to halve size the current number. The interval between one number and the next or between one number and the previous is always based on the power of two. Keep in mind that this half-and-double size concept is the very basic of subnetting as you will later find out. 1 x2=2 2 x2=4 4 x2=8 8 x 2 = 16 16 x 2 = 32 32 x 2 = 64 64 x 2 = 128 128 x 2 = 256 | half the size | A | | | | | | | | V | double the size | In terms of subnetting, 1 = /32 = 255.255.255.255 2 = /31 = 255.255.255.254 4 = /30 = 255.255.255.252 8 = /29 = 255.255.255.248 16 = /28 = 255.255.255.240 32 = /27 = 255.255.255.224 64 = /26 = 255.255.255.192 128 = /25 = 255.255.255.128 256 = /24 = 255.255.255.0 In terms of IP address quantity /32: /31: /30: /29: /28: /27: 2^0 2^1 2^2 2^3 2^4 2^5 = = = = = = 1 2 2 2 2 2 = x 1= x 2= x 4= x 8= x 16 = 1 IP address within the subnet 2 = 2 IP addresses within the subnet 4 = 4 IP addresses within the subnet 8 = 8 IP addresses within the subnet 16 = 16 IP addresses within the subnet 32 = 32 IP addresses within the subnet of of of of of /32 /31 /30 /29 /28 = = = = = 2 2 2 2 2 /26: 2^6 = 2 of /27 = 2 x 32 = 64 = 64 IP addresses within the subnet /25: 2^7 = 2 of /26 = 2 x 64 = 128 = 128 IP addresses within the subnet /24: 2^8 = 2 of /25 = 2 x 128 = 256 = 256 IP addresses within the subnet Side Note: Some people refer the xxx.xxx.xxx.xxx notation as octet (8-based number system). There are four octets in the notation. To separate the octets, there are dots between them. I call them 4-tuple octet. For /24 to /32 subnets, the 1st three octets remain the same. The only octet that changes is the 4th octet (the last octet). Subnet Mask Subnetting always relates to something called Subnet Mask. Subnet Mask is the way an IP address represent which subnet it is under. To show you how a subnet mask looks like, let's review the previous table. 1 = /32 = 255.255.255.255 2 = /31 = 255.255.255.254 4 = /30 = 255.255.255.252 8 = /29 = 255.255.255.248 16 = /28 = 255.255.255.240 32 = /27 = 255.255.255.224 64 = /26 = 255.255.255.192 128 = /25 = 255.255.255.128 256 = /24 = 255.255.255.0 As mentioned, there are octets separated by dots that I called 4-tuple octet. This 4-tuple octet is commonly known as Subnet Mask. Another format of displaying subnet mask is by using CIDR (Classless InterDomain Routing) format. Where the 4-tuple octet format is shown on the above table rightmost column, the CIDR format is shown on the above table leftmost column. To clarify, check out following table. IP Address Quantity mask Within Subnet 1 2 4 8 16 32 64 128 256 CIDR format subnet mask (Prefix) /32 /31 /30 /29 /28 /27 /26 /25 /24 4-tuple octet format subnet 255.255.255.255 255.255.255.254 255.255.255.252 255.255.255.248 255.255.255.240 255.255.255.224 255.255.255.192 255.255.255.128 255.255.255.0 Later you will find out that CIDR format is based on the subnet mask binary format where the number behind the / represents how many bits are set to 1 (one) contiguously from the left. You will also learn this relationship between subnet mask and IP address quantity within the subnet from the subnet mask binary format. As a note, the CIDR format subnet mask as shown is known as prefix. The higher the number, the prefix is called longer prefix. The lower the number, the prefix is called shorter prefix. As an illustration, a /26 is a shorter prefix than /28 but is a longer prefix than /24. Now let's see how a subnet mask determine the fitting in of an IP address into a subnet. Example #1 IP Address: 192.168.0.4 Subnet Mask: /31 (255.255.255.254) From the table above, /31 informs that there is a network that consists of two IP addresses; 192.168.0.4 and 192.168.0.5. The 192.168.0.4/31 is the 1st IP address of the network. Example #2 IP Address: 192.168.0.4 Subnet Mask: /30 (255.255.255.252) Refer to the table, /30 shows that there is a network that consists of four IP addresses; 192.168.0.4 to 192.168.0.7. The 192.168.0.4/30 is the 1st IP address of the network. Example #3 IP Address: 192.168.0.4 Subnet Mask: /29 (255.255.255.248) Using the above table, /29 reflects that there is a network that consists of eight IP addresses; 192.168.0.0 to 192.168.0.7. The 192.168.0.4/29 is the 5th IP address of the network. Example #4 IP Address: 192.168.0.4 Subnet Mask: /32 (255.255.255.255) Based on the above table, /32 points that there is a network that consists of a single IP address; 192.168.0.4. The 192.168.0.4/32 is the 1st and the last (the only) IP address of the network. What IP Address Represents in IP Network In general, an IP address represents a machine within certain IP network. This machine can be any machine that understand IP address such as PC, printer, server, router, firewall, X-Box, and Playstation. In networking term, such 254) Number of IP address within the subnet: 2 Number of IP address available for host: 2 IP Address range: 192.0/29 IP Address: 192. 192. then Host A uses its IP address to communicate with Host B.0/31 then you should be able to tell the following IP Address: 192. let's say you have the following * 192.168. Host B in this communication back to A also uses its IP address.0.168. meaning there should be a reply communication from Host B to A.machine is sometimes called a host.0 Subnet Mask: /31 (255. This communication is a two-way communication.0. you can tell the following * Number of IP address within the subnet * Number of IP address available for host * IP Address range of the subnet Later on.0. you can also tell the following from subnet mask of specific IP address * The first IP address within the subnet (called Network ID IP address) * The last IP address within the subnet (called Broadcast IP address) In terms of IP network communication between two hosts say Hosts A and B.0 When you have the following * 192. What Subnet Mask Represents in IP Network In general. a Subnet Mask represents how large an IP network is.255.168. you should be able to tell the following IP Address: 192.255.168.168.255.0.0.168. When a host (let's say Host A) in IP network needs to communicate with other host of some IP network (let's say Host B).168.0.0. From subnet mask of specific IP address.1 Similar result with the following * 192. A can tell if itself is within the same network as B or not just by checking itself IP address and subnet mask.0. On later discussion. Number of IP Address Within A Subnet By referring to the previous table.168.0 Subnet Mask: /32 (255.168.0.255) Number of IP address within the subnet: 1 Number of IP address available for host: 1 IP Address range: 192.255.0/32 Based on the above.0 .0. you will see how this ability plays important part in IP network communication. 1.6. 192.0.0.0.0.0/31 network can hold two hosts with IP address of either 192.168.0.168.168. 192. For a list of host number certain network can hold.------------+ | | +---------------------------------------------------------------------------------.Subnet Mask: /29 (255.168..255. To describe..0.7 As a note that when the available IP address is implemented into machine or host.0. note that 192.. The second example shows that 192.0.168.168.248) Number of IP address within the subnet: 8 Number of IP address available for host: 8 IP Address range: 192.------------+ A | 1st IP address (192.168.0/29 network can hold eight hosts with IP address of either 192.0. 192.0/24 as a long piece of wood where each end represents the 1st and the last IP addresses.168.168.255/24) A | Last IP address When you break up a /24 network into two equal sub-networks.0.0.255/24 respectively.0.1.0.0..0.0. to 192.168.168.0.6.168..0. you can see the 192.-----.0/24 network.0.. 192.-----.0..168. Following is the break down. /24 +---------------------------------------------------------------------------------. The third example shows that 192. The first example shows that the 192.254.168.168.0/24) ( 192. you may wonder how to create such table that shows specific IP address belongs to specific order number of a network or to determine the IP address range available for host by just looking at the subnet mask..7.168.0. .0.168.0.0.0..0.0/32 network can hold single host with IP address of 192.1.168.0.255.168.0.168. 192.1. 192.2. . then there are the following understanding. 192. 192.0.. or 192. .0.0 or 192.168.168. refer to the following /32: /31: /30: /29: /28: /27: /26: /25: /24: 1 IP address within the subnet = 1 host can exist within the subnet 2 IP addresses within the subnet = 2 hosts can exist within the subnet 4 IP addresses within the subnet = 4 hosts can exist within the subnet 8 IP addresses within the subnet = 8 hosts can exist within the subnet 16 IP addresses within the subnet = 16 hosts can exist within the subnet 32 IP addresses within the subnet = 32 hosts can exist within the subnet 64 IP addresses within the subnet = 64 hosts can exist within the subnet 128 IP addresses within the subnet = 128 hosts can exist within the subnet 256 IP addresses within the subnet = 256 hosts can exist within the subnet The Size Doubling and Subnetting Referring to the previous examples. In this following illustration. Referring to the above host IP address availability table.168. 192. let's start with 192.168.0/24 network consists of 256 IP addresses.168.0/24 and 192. note that you . from 192.168.255. 0/24.--------------+ 1st half (1st /25) 2nd half (2nd /25) A AA A | || | 1st IP address (192. 192.0. Let's call these four pieces as 1st. You now have four /26 networks when you break up a /24 network into four equal-size networks.0/26 (1st quarter).0.128/25 (2nd half).1.0.168. a /25 network is double the size of /26 network.128/26 (3rd quarter). then the four /26 networks off the /24 network are 192.128/25) Similarly.0. breaks up a /25 network into two equal-size networks gives you two /26 networks.---------------+ | || || || | +-----------------------+ +--------------------------+ +----------------------+ +-------.168.---------------+ 1st /26 2nd /26 3rd /26 4 th /26 A AA AA AA A .0.168.255/24) || Last IP address (192. /24 +---------------------------------------------------+ +---------------------------------.0. from 192..0.0. Therefore the original long piece of wood of a /24 now becomes four pieces of equal size. . Following is the illustration. from 192. then the two /25 networks off the /24 network are 192.168.0. and 4th quarter respectively.255.168. Since the /24 network is 192.0. Since the /24 network is 192. 192.0/25 network.168. Using the long piece of wood illustration.have two /25 networks. 2nd. Let's call the pieces as the 1st half and the 2nd half.168.. 192.0.. a /24 network is double the size of /25 network.127/25) 1st IP address (192.--------------+ | || | +---------------------------------------------------+ +---------------------------------.168. Each of the four /26 networks consists of 64 IP addresses.168.. and 192.. to 192..0.0/24.0. 192. the long piece of wood of a /24 breaks into two pieces of equal size. each piece of wood of a /25 breaks into two pieces of equal size. .192/26 (last quarter). In other words.128.0. The 1st half. 192.127.168.168. The 2nd half. consists of also 128 IP addresses.168.168.129.0. to 192.168.168.168.0.0.168.0. Using the long piece of wood illustration.64/25 (2nd quarter)..0/25) || Last IP address ( 192..168. /24 +-----------------------+ +--------------------------+ +----------------------+ +-------.0. consists of 128 IP addresses. Following is the illustration.128/28. 192. In other words.0.168. 3rd.168.0.0/25 (1st half) and 192. 0.168.192/26) The same logic continues where /26 network is double the size of /27 network (or /27 network is half size of /26 network) and /31 network is double the size of /32 network (or /32 network is half size of /31 network). following shows how larger subnet size correlates to smaller subnet size.168.168.168.191/26) | ( 192. As mentioned earlier. the interval between one number and the next or between one number and the previous is always based on the power of two. /24 = 2 x /25 = 4 x /26 = 8 x /27 = 16 x /28 = 32 x /29 = 64 x /30 = 128 x /31 = 256 x /32 How did this half-and-double size concept come form? Let's review the previous table to find the answer.168.168.255/26) || | | Last IP address 1st IP address 1st IP address 1st IP add ress (192. To sum up the understanding.0.64/26) (192.0.128/26) (192. /32: /31: /30: /29: /28: /27: /26: /25: /24: 2^0 2^1 2^2 2^3 2^4 2^5 2^6 2^7 2^8 = = = = = = = = = 1 2 2 2 2 2 2 2 2 = 1 IP address within the subnet 1 = 2 = 2 IP addresses within the subnet 2 = 4 = 4 IP addresses within the subnet 4 = 8 = 8 IP addresses within the subnet 8 = 16 = 16 IP addresses within the subnet 16 = 32 = 32 IP addresses within the subnet 32 = 64 = 64 IP addresses within the subnet 64 = 128 = 128 IP addresses within the subnet 128 = 256 = 256 IP addresses within the subnet of of of of of of of of /32 /31 /30 /29 /28 /27 /26 /25 = = = = = = = = 2 2 2 2 2 2 2 2 x x x x x x x x Note that the concept of half-and-double size is based on the binary system where you can only double the network size or break up the subnet into two equal size of smaller networks as shown on previous table. The 4 /26 networks: 1.63/26) (192. Using the same logic as presented.0. Subnetting /24 network into * * * * 2 equal network size makes 2 of /25 networks 4 equal network size makes 4 of /26 networks 8 equal network size makes 8 of /27 networks 256 equal network size makes 256 of /32 networks When the /24 network is 192.168.168. which introduces the very basic of subnetting logic.0. let's recap.0 .0/24.0/26) || (192.0.| || || || | 1st IP address || Last IP address | Last IP address | Last IP address (192.0. then following are how the smaller subnets look like.0.168. 192.127/26) | (192.168.0/26 .0. 168...168.0. .168.255 If let's say you only need nine subnets off 192.65.192/26 IP Address: 192.0.192) Number of IP address within the subnet: 64 Number of IP address available for host: 64 IP Address range: 192..255.0.63 2.168.255 The 32 /29 networks 192.0. 192...0. .129.0.255.0.168.248 ..80/28. 192. 8 (as of 8 x /27)..0...168.0 Subnet Mask: /26 (255. 192.168. then you can consider the 16 of the /28 subnet size * You could then take the 1st nine subnet out of the 16 subnets available * The nine subnets you take are 192.255.0. then following are the considerations.1.168.168.240 .0.64/26 IP Address: 192.168. .168. to 192.0. and so on * Nine is higher than 8 and is lower than 16 * To accommodate the nine subnets.8 .192) Number of IP address within the subnet: 64 Number of IP address available for host: 64 IP Address range: 192.248/29 = 192. .168..64/28.128 Subnet Mask: /26 (255.0 /29 = 192..192) Number of IP address within the subnet: 64 Number of IP address available for host: 64 IP Address range: 192.16 .0.255.0.48/28.168.192 Subnet Mask: /26 (255.7 192. 192.255.0/24.16 /29 = 192.0.0. 4 (as of 4 x /26).IP Address: 192.64 Subnet Mask: /26 (255.127 3.128.0.168.240/29 = 192.0.0..15 192.0.168.0.192) Number of IP address within the subnet: 64 Number of IP address available for host: 64 IP Address range: 192.0.168.0 .0.168.64.247 192.168.168.192.168.255.0.0. . 192. * All subnets are in equal size * Subnet quantity are only in two-power form which are 2 (as of 2 x /25).0.255.168...168.191 4.193.192.16/28.192.168.192. 192. to 192. 192.8 /29 = 192.168.0.0.0. 192.23 .168.168. .0.0.0.0.168.168.168.168. 192.168.168.0.0. 192.0.168.0.32/28. 16 (as of 16 x /28).0.0. to 192.255.192.168..168.0. 192.0. 192. to 192.128/26 IP Address: 192. 192.168.168.168. .0..168.0.192.168.0/28.168. 224 . Let's say you are given 192.168.0. You need to determine the following * range of IP addresses within the subnet * order number of 192.96 .0.192.192.168.79 192.0.192. This is one of key in easy subnet calculation using decimal number system with binary number system in mind.0/24 network.143 192.168.176 .168. you need to calculate starting from /24.15 The remaining /28 network should be the following 192.0.0.0.112 .xxx.0.168.128 .0.67/28 as part of larger 192.192.168.168.168.168.0.192.168.168.0.168.95 192.0.0.0.16 .32 .0. Here is illustration.0.223 192.192. When you calculate subnet larger than /24.63 192. where dots are used to separate one octet from another. Any /25 or smaller subnet calculation must refer to the larger /24 network.0.67 IP address within the subnet You start by seeing 192.144 . Therefore the 1st /28 should be the following 192.168. In other words.168.192.0 .192.168.168.0.192.192 .112/28.191 192.0.64 .0.168. Another key to such easy subnet calculation is utilizing the octet. 192.192. Referring to the previous table.127 192.0.175 192.168.168.208 .192.48 .0.168. In calculating IP address within /24 subnet or smaller for example. There are four octets on both IP address and subnet mask representation. Specifically with /25 or smaller subnet.168. Start working from the last octet and work up the one previous octet when necessary. note that only the last octet is changing as mentioned previously. In other words.67/28 network.0.0.0.168.96/28.0. must refer to the last octet. you will then consider the 3rd octet (and the rest) as necessary.xxx).0.0. /28 = 2^4 = 16 IP addresses.0.168.168.168. In math.159 192.192.168.0.192.0.168.168.0. you should see the /25 or smaller subnet as part of larger /24 network.168.168.192.0.160 .xxx.0 IP address (the 1st IP address) to create a list of smaller /28 networks off a larger /24 network.0. and 192. you have to start calculating from 192.192.47 192.128/28 Octet and Subnet Calculating As you may notice.80 .0. octet can be seen as "summary" of binary numbers.168.192. or in other word.168.0.168.0.239 .168.207 192.0.168.168.111 192.168.0.168. IP address and subnet mask are presented in form of octet (the xxx.31 192. 0.64 and 192.168.48/28 192.96/28 192.0.168.0. Illustration as follows.168.168.0.168.168.64 .168.0.79 = 192.240 .0.2nd.168.168.168.192.192.168.168.0.0.224 .0.0.0.160/28 192.79 or within 192.168. By referring to the 1st IP address of each /28 subnet.168. while the 1st three octets are constant.0.224/28 192.31 = 192.80/28 192.192 .168.168.168.223 = 192.0/28 Octet 1st.168.0.0.0.208 .0.208/28 192.168.168.0.168.15 = 192.168.128/28 192.80 .168.168.168.168.192.4th 192. you see that 192.0.168.0.168.0.0.168.0/24 network.168.168.192.0.32/28 192.3rd.32 .16 .144 .3rd.192. where the 192.168.0. 0 16 + ---16 Next Subnet: 192.0.168.168.48 .0.191 = 192.168.0.2nd.0.0.0.0.168.175 = 192.0.0.192.143 = 192.192.64/28.0.160 .67/28.0.0.0.168. 0 .0. you present those subnets as follow 192.192.255 = 192.64/28 192.159 = 192.168.4th 192.67 is between 192.0/28 192.95 = 192.128 . 1st Subnet: 192.168.79.0. Therefore the IP address range is 192.0.192.64 . Recall the question of finding the IP address range of 192. you can add 16 to the 4th octet of 1st IP address of the subnet to find out the next 1st IP address of the next subnet.192.192/28 192.168.127 = 192.0.0.168.168.0.168.0.168.112 .47 = 192.0.16/28 2nd Subnet: 192.168.111 = 192.168.168.16/28 192.176 .0. 0 .0.168.16/28 Octet 1st.0.0.168.192.240/28 Later on. Referring to the above table.192.168.168.0.168.192.239 = 192.0.240 .63 = 192.168.0.0.192.176/28 192.192.0.168.168.0.144/28 192.207 = 192.0.255 which makes up the entire 192.192.67 is the 4th IP address.168.0.0 .0.168.192.168.0.0.16 16 .168.192.168.168.112/28 192.168.168.168. you will learn that the 1st IP address as the reference is called Network ID. Note: Since there are 16 IP addresses within one subnet.0.0.192.0.96 . 4/30 = 192.168.4 .168. Following is a list of smaller subnet groups to make up the entire Class C network /24 = 2 x /25 = 256 x /32 /25 = 2 x /26 /26 = 2 x /27 /27 = 2 x /28 /28 = 2 x /29 /29 = 2 x /30 /30 = 2 x /31 /31 = 2 x /32 = 4 x /26 = 8 x /27 = 16 x /28 = 32 x /29 = 64 x /30 = 128 x /31 = = = = = = 4 4 4 4 4 4 x x x x x x /27 /28 /29 /30 /31 /32 = = = = = 8 8 8 8 8 x x x x x /28 /29 /30 /31 /32 = = = = 16 16 16 16 x x x x /29 = 32 x /30 = 64 x /31 = 128 x /32 /30 = 32 x /31 = 64 x /32 /31 = 32 x /32 /32 Supernetting As noted on previous discussion.0. the result is the basically the same as the 192. you have a choice to present them in the form of 192. In the previous 192.0.168. Note that 192. 192. The result network is called supernet.0.0. This combining process is called supernetting.0.168. This time you have four /30 networks.0/30.168.0/24 covers the same IP addresses as the 32 of 192.168.168.168.0.168. Let's review another illustration.32/28 Later you will learn how to find such specific subnet quickly using some tips and tricks.0.0/24 can be seen as groups of smaller /28 subnets (16 of /28 subnets).0.192.0.0. Now let's say you have 32 of 192.x/29 to the same device or audience. When you see such from the other way around. 192. you can create a list of /28 networks to make up a larger 192. you have a concept of seeing a Class C network (full /24) as either groups of single IP addresses (256 of /32 subnets) or groups of smaller subnets.168.168.168.0/24 network. When you combine the entire /28 networks.168.168.0.+ ---32 Next Subnet: 192. you can create a list of /28 networks to make up a larger 192.0/24 network. As noted.0.0/24 network.0.168. the 192. When you present the 32 of 192.0.168.20/30.0.0/28 network illustration.168.0.0.0/24.7 .168.x/29 networks.0/28 network illustration.16/30.0. Group of Multiple Single IP addresses or subnets Let's review the previous 192.0/24.192.0/24 network.0.x/29 networks.x/29 to become larger 192. 192. When you present the 32 of 192.3 192.0.168.168.0.0.168.168. You then have the following IP address ranges 192.0.168.0.168.168.168. and 192.0.4/30.0.0 .x/29 networks in the form of 192. you combine the whole 32 of 192.0/30 = 192. 16 .0.192.0.0. you have the following IP address ranges 192.168. A good thing of supernetting is summary without losing details.0.8/29) 192.8 .24/29) Therefore to supernet the four /30 networks.0. 192.23 In simpler form.192.192.23 = 192.0.168.168.168.168. Following is the reason. Ethernet Host A Host C Host E | | | +------+-------+-------+--------+ | | Host B Host D Token Ring Host A Host C | | +--------------+ | | Host B ---+ +--.31 (= 192.168.168.16/29 You then have a choice to supernet the four /30 networks to become two larger /29 networks.168.168.16 .0/27.192.7 = 192.168.168.0.168.168.0/29 192.168.20/30 = 192.0 .0.0. Example of broadcast network are Ethernet and Token Ring network.Host D .31 You don't have the following range 192.168.168.168.192.168.0.0. dealing with supernetting means conserving resources such memory and CPU utilization.16/30 = 192.168.0. In network device perspective.192.0.24 .0.192.0. Note that you cannot supernet the four /30 networks to become a single larger /27 network of 192.0.19 192.20 .168.0. you only have a choice to have two larger /29 networks.0/27 = 192.0.0.15 (= 192.0. Following is illustration.0. Some discussions »[HELP] Why use the no auto-summary command for EIGRP Broadcast Network A loose understanding of broadcast network is a network that consists of multiple IP addresses (multiple machines).168.192.0.0 .168. 0.0.168.0.0.168. This broadcast mechanism requires the communication partner IP address (the Layer 3 info) or the communication partner MAC address (the Layer 2 info) before starting the mechanism.32/28 = 192.0. As example. Network ID and Network Broadcast IP Addresses In broadcast network.168. Network ID is an IP address that is used to identify specific subnet. usually the broadcast IP address is the last IP address. When there is a special situation as you find out later on.| | +------+-------+ | Host E Since there are multiple IP addresses within the network. one IP address broadcasts its communication partner IP address and expect to receive a reply from the partner with its MAC address.0.168.31 .48 192. Normally Network ID and Network Broadcast IP addresses are not allowed to be used by hosts in broadcast network since such IP addresses are reserved for ARP/RARP processes.168. the network ID IP address of 192.15 .8/29.0. there is a need to send broadcast to find either IP address of specific MAC address. To broadcast.0/29 subnet is the 192.168.168. In a subnet. In a subnet.64 . the broadcast IP address of 192. the network utilizes an IP address.0.0.0/28 subnet table 192. there is also a term called Network ID.0. it starts to communicate with its partner using MAC address.168.168.63 .16/28 = 192.168.16 192.0.32 192.0/28 = 192. Network ID and Network Broadcast Subnets Let's review the previous 192.0.0.192.0. This IP address is called broadcast IP address. or to find MAC address of specific IP address. usually the network ID is the 1st IP address. As example.0.168.7/29. Once the IP address receives its communication partner MAC address. the Network ID and Network Broadcast IP addresses could be usable by hosts.48/28 = 192. This process is called ARP (Address Resolution Protocol) or RARP (Reverse ARP).168.192.168.0 192.0.47 .79 = 192.192.0.168.8/29 subnet is the 192. Subnet Zero.168.0. To find its communication partner within a broadcast network.192. In broadcast network.168. there must be a mechanism to properly select communication partner IP address when one IP address need to communicate with other IP address within the network.0.168.168. The mechanism is called broadcast.192.0.168.168.64/28 . 168.240/28 Similar to the concept of Network ID and Network Broadcast IP addresses.168.144 .95 = 192.111 = 192.80 .192.0.192/28 192.168.208/28 192.192.240/28 As a note.192.192.192.160/28 192.128 .96 .0.192.168.176 .168.192.112/28 192.0.0.0. Beside a broadcast network.0.192. Subnet Zero itself refers to the subnet of all 0 and/or subnet of all 1 in binary system perspective.0.168. Subnet Zero refers to the Network ID (the all 0) and Network Broadcast (the all 1) subnets.80/28 192.168. Here is an illustration Broadcast Network Host A Host C Host E | | | +------+-------+-------+--------+ | | Host B Host D .168.0.0.168.0.168.0. On today's network.168.0.168. network ID and broadcast IP addresses are used to find IP address within the broadcast network to be able to have one IP address to communicate to the IP address in question.0.0.168. Network ID subnet is the 1st subnet and Network Broadcast subnet is the last subnet.0. it is now permitable to use the Network ID and Network Broadcast subnets for host IP address assignments.191 = 192. most network are Subnet Zero applicable.0.0. Unlike broadcast network where there are multiple IP addresses.96/28 192.168.159 = 192.168.168.168.224 .239 = 192.0. there is also a point-to-point network.0.0.223 = 192.0.0.0.0.168.255 = 192.0.207 = 192.168.0. In the beginning of subnetting technology.240 .0.192.168. In other words.0.208 .0. As mentioned earlier.168.168.112 . Broadcast and Point-to-Point Networks Network ID and broadcast IP addresses usually exist in broadcast network. However it is always a good idea to confirm if Subnet Zero is permitable to use or not on specific network.175 = 192.168.0.168.168.168. most of the time Subnet Zero usage is assumed. When you have to assign IP addresses and/or subnet on today's network. With newer technology.168.0.192 .143 = 192. using Network ID (or Subnet Zero) and Network Broadcast subnets are not allowed with similar reason of not allowing Network ID and Network Broadcast IP address usages.168.168.128/28 192.160 .176/28 192. On the previous table. Network ID Subnet : 192.168.0/28 (also called Subnet Zero) Network Broadcast Subnet: 192.192.0.224/28 192.192.168.0.168.192.0.168.0.168.168. point-to-point network only consists of two IP addresses.144/28 192.127 = 192. 0.0.168.2.3 Network ID : 192. When you have a broadcast network that consists only two IP addresses.168.168.168.0/30 IP Address : 192. there are only two actual IP addresses within 192.168. It is possible however to have broadcast network consisting of only two IP addresses.168. In 192. you then have a choice to keep it as broadcast network or convert it to a point-topoint network.0.168.252) Number of IP address within the subnet : 4 Number of IP address available for host: 2 IP Address range : 192.0.1 and 192. Since there are only two IP addresses in the network. Note that when someone says broadcast network.2.3 IP Address range for host : 192.Point-to-Point Network Host A ------.1. the 192.Host B In a point-to-point network. 192.0/30 and network broadcast is 192.0 Network Broadcast : 192. Should you decide to convert.168. If a broadcast network only consists of two IP addresses.168.255. the other IP address must be the one that needs to be communicated with.255.168.2 As you can see.0.1 and 192. it is basically point-to-point network.168.0. The network ID is 192.0.168.168.0 .0.0.2/31 for future use should you have more point-to-point network.0. then you can subnet 192. the actual IP addresses that can be the host's IP addresses are the remaining after IP addresses reserved for network ID and network broadcast.0.0. You then have an option to convert the network to be point-to-point network.0 Subnet Mask : /30 (255.2/31. In a broadcast network.0.0/30 network. Let's say we have 192. the one IP address no need to select its communication partner.192. You can then keep the 192.0. there should be no need to use network ID and broadcast IP addresses. The network is a broadcast network. the person usually means that the network consists of more than two IP addresses. one is 192.168.168.0.0.168. the available IP addresses that can be the host's IP addresses are 192.168.168.0.0/31 for these two hosts to communicate with each other. Therefore this broadcast network is somewhat point-to-point network. Here is the breakdown 192. When one IP address needs to communicate with other IP address.0/30.168.0/30 network.168. Here is the breakdown .0.3/30.0.0.0/30 into two /31 networks.0. Following is an illustration.0/31 and another is 192.168.168. You can use 192.0. 192.255.11111111.0.255.255.11110000 255.0.255.0/30 into 2 equal network size makes 2 of /31 networks 1.0.255. .0.168.0.. 192.0.0.224 = 11111111.168. 4th 255.255.11111111. 192.2/31 = 192. 192.1 192.11111000 255.0.255. 2nd /24: /25: /26: /27: /28: /29: /30: /31: /32: .* Subnet 192.0 subnetting process and host quantity availability within as follows Subnet IP Address available within the subnet 192.255.11111111.4th 1st .0.252 = 11111111..0/31 = 192.255.11111111.240 = 11111111.11000000 255.0.1 2. 192.168.11111111.168.255.0/31: 192.0.168.11111111.10000000 255.0/31 and keep 192. 3rd .11111111..0.0.168.0.0.11111110 255.255 = 11111111.168.11111111.255.168.255.168. 192.0.11111111.0.255.0.3 * Use 192.168. 192.0/32: 192.11111111.0/30: 192. let's review 192.11100000 255.0 192.2nd.168.168.168.255.11111111.3 .0.2/31 for future use * Convert hosts' interface to be point-to-point network member from broadcast network member * Assign 192.11111111.3rd.11111100 255.11111111.0.0.11111111.192 = 11111111.2.0.255.0.248 = 11111111.11111111 Host quantity available within specific subnet /32: /31: /30: /29: /28: /27: /26: /25: /24: 1 IP address within the subnet = 1 host can exist within the subnet 2 IP addresses within the subnet = 2 hosts can exist within the subnet 4 IP addresses within the subnet = 4 hosts can exist within the subnet 8 IP addresses within the subnet = 8 hosts can exist within the subnet 16 IP addresses within the subnet = 16 hosts can exist within the subnet 32 IP addresses within the subnet = 32 hosts can exist within the subnet 64 IP addresses within the subnet = 64 hosts can exist within the subnet 128 IP addresses within the subnet = 128 hosts can exist within the subnet 256 IP addresses within the subnet = 256 hosts can exist within the subnet As illustration.11111111.168.0 = 11111111.168.255.168.1 with /31 subnet mask to another host Subnetting from Binary-Number System Perspective * Wildcard Let's review the following tables Subnet mask in their octet and binary form OCTET 1st.168.11111111.168.11111111.00000000 255.168.254 = 11111111.11111111.1.0 with /31 subnet mask to one host and assign 192.168.128 = 11111111.255.168.0.255. 11111111.0.255 = 11111111. Since this "changing" number varies (could be any number).7 /30: 255.11111111.168. the first three octets are "fixed" while the last octet "changes".11100000 = 192.168.11111111.168.1 /32: 255. 192.1.0.0.168.4th 1st . 192.1.0.168.168.168.1.0.0.0.255.11111111.0.0..168.1.168.254 You may notice that the IP Address available within the subnet are within the range of its 0 (zero) bit of the binary form.127 /26: 255.168. 192. . 4th /24: 255.. 192.168..1.0.63 /27: 255.31 /28: 255.168.1.0.0.255.168.0.255.1.0.168.0.255.0..0/29: 192.0/25: 192..11111111.168.11111000 = 192.0.168.168. From the 4-tuple octet format perspective.11111111.. ..0.255. depending on how large the subnet is... 192.0.240 = 11111111..0. 192..255...0.0.255.0.11111100 = 192. .11111111. . This "changing" number as mentioned can be any number.. .168.2nd. . 192..0.255. the 0 (zero) bit of the binary form (or the last octect of the 4-tuple octet format) represents a set of possible number of the last octet of the subnet. 192.0 With similar understanding.11111111. the 1 (one) bit of the binary form represents a set of "fixed" number while the 0 (zero) bit represents a set of "changing" number. 192.168.168.11111111.00000000 = 192.11111111.0.255.0. .0. 192.0...3 /31: 255.63 192.168. As illustration. often the changing number is called wildcard. 192.0..168.31 192.0/26: 192.0.. .255.0.11111111.0.11111111. .. 192.0. .168.248 = 11111111. the changing number can be ranging from 0 to 127 when the subnet is /25 size.0.0.0/28: 192. .255. 2nd IP Address available within the .0..192 = 11111111.168. Following is the illustration.11111111.168.3rd..1.168.224 = 11111111.168. Note that this understanding of wildcard is essential when you delve into Wildcard Subnet Mask as discussed in the following FAQ.0.1.192.168. 192.. 192.11111111. Quickly Convert CIDR Format To Dotted 4-Tuple Octet Format .11111111.0.0 = 11111111.. 192. 3rd .0. .1. .0.0.168.11111111 = 192.0.10000000 = 192.11111111.168.255. .7 192..168.127 192.0.168.. In other words.0. 192.0.168. and the changing number can be ranging from 0 to 31 when the subnet is /27 size.1.128 = 11111111. .168.15 192.1...255. 192. 192.0.168. 192. 192.254 /25: 255.0.15 /29: 255.0.0.0 subnetting process OCTET subnet 1st. .0.0.11110000 = 192.168.11111110 = 192.11111111.252 = 11111111.. 192. 192.168.255.168.0.0. . . 192.168.168.254 = 11111111.0.255.255. 192.0.0..11111111. .168. »Cisco Forum FAQ »Quick and Easy Wildcard (Inverse) Subnet Mask Some Tips and Tricks 1.1.0.168.168.0/24: 192. 192.. .255.168.168.0/27: 192. 192.0.255.168.0.168. 192.0.168.168.0.0. 192.11000000 = 192.0.. Here are the key points. Let's say you have /26 CIDR subnet mask and you need to know how the subnet mask looks like in the dotted 4-tuple format.224 64 IP addresses = 32 x 2 = /26 = 255.128 255.255.255. Also keep in mind the IP address amount availability within each subnet.255.2 = 252 252 .255.255.255.32 = 192 192 .255.255.255.248 255.255.255. there are two IP addresses within /31 CIDR (double of 1 in /32 CIDR). Following is the table 1 IP address = 1 = /32 = 255. Also mentioned. Start with the longest prefix.255.0 As mentioned. The next bigger subnet size is always double of the current subnet size.255.255.248 16 IP addresses = 8 x 2 = /28 = 255. * The /32 always match with 255 of the last octet on the dotted 4-tuple .255.128 = 0 The subtraction result is always the last octet next number.255.255.4 = 248 248 .252 8 IP addresses = 4 x 2 = /29 = 255.240 255. only the last octet changes.0 = 255 255 . the 1st three octet in the dotted 4-tuple always stays the same in /24 and longer prefix.255.192 128 IP addresses = 64 x 2 = /25 = 255. In other words.255.255 2 IP addresses = 1 x 2 = /31 = 255.255.255. which /32 CIDR.254 4 IP addresses = 2 x 2 = /30 = 255.16 = 224 224 .128 256 IP addresses = 128 x 2 = /24 = 255.255.0 Now let's only consider the last octet on the dotted 4-tuple format. Note the change on the last octet in the following table /32: /31: /30: /29: /28: /27: /26: /25: /24: 255 .255.255.252 255.64 = 128 128 .255 255.1 = 254 254 . In other word.255.255.255.255.8 = 240 240 .255.255.255. You can apply this to convert CIDR format to dotted 4-tuple format quickly.254 255.255.224 255.192 255.255. there is only one IP address within /32 CIDR.255.240 32 IP addresses = 16 x 2 = /27 = 255.Let's review the following CIDR and dotted 4-tuple octet format comparison /32 /31 /30 /29 /28 /27 /26 /25 /24 = = = = = = = = = 255.255.255. 255. hence it is called octet (the 8-bit numbering system).previous number of IP = (8 x 2) IP addresses) = 224 (the new current last octet) /26 CIDR: 224 (the current last octet) . As a note.255. .previous number of IP (16 x 2) IP addresses) = 192 (the new current last octet) address address address (= 2 address (= 4 address (= 8 address (= 16 address (= 32 = Referring to the build-up table. the 256 range is based on 8-bit system.224 subnet mask. the 255 is the last number within 256 range.previous number of IP (= 1 IP address) = 254 (the new current last octet) /30 CIDR: 254 (the current last octet) .previous number of IP = (4 x 2) IP addresses) = 240 (the new current last octet) /27 CIDR: 240 (the current last octet) . /32 CIDR: 255 (the current last octet) .xxx. Say you have 255. let's review the following.192 Since the 1st three octet are always 255.previous number of IP (= 0 IP address) = 255 (the new current last octet) /31 CIDR: 255 (the current last octet) . Let's review the following table CIDR The range Last octet on Number of IP address within octet the 4-tuple within the subnet /32: 256 255 = 1 /31: 256 254 = 2 /30: 256 252 = 4 /29: 256 248 = 8 /28: 256 240 = 16 /27: 256 224 = 32 /26: 256 192 = 64 /25: 256 128 = 128 /24: 256 0 = 256 Referring to the table. where 0 (zero) is the 1st number.255. Quickly Find How Many IP Address Within Specific Subnet For the next tip. You may notice that in dotted 4-tuple format. only consider the last octet * In /24 CIDR or longer prefix (within Class C).255.224 subnet mask and you need to know how many IP addresses within the subnet. which is 255 * /32 only has one IP address * The next bigger subnet has always double size of current subnet Using these knowledge.192 2.previous number of IP = (1 x 2) IP addresses) = 252 (the new current last octet) /29 CIDR: 252 (the current last octet) . you know now that there are 32 IP addresses within the 255.255.* To convert /24 CIDR or longer prefix to the dotted 4-tuple. the 1st three are octet always the same. then the dotted 4-tuple format equivalent to /26 CIDR is 255.previous number of IP = (2 x 2) IP addresses) = 248 (the new current last octet) /28 CIDR: 248 (the current last octet) .xxx.255. you can start creating your own build-up table as follows. you know now that /26 CIDR is equal xxx. 3.255.255. which is the range within octet. the 1st 29 bit are set to 1 and the remaining bit are set to 0.11110000 255.255.3rd.252 = 11111111.254 = 11111111.255.11111111.248 = 11111111.255. On the /29. In other words.11111111.255.0).4th 1st .11100000 255.2nd. To subnet the /24 into /29.11111111.11111000 255.11111111.255.240 = 11111111.11111111.10000000 255.255.11111111. In the binary format. the subnet mask is always the reverse bit of the IP address number from binary format perspective. The full Class C network is /24 CIDR (255. Note that the sum between the octet and number of IP address is always equal to 256.11111111.255. the set-to-one bits are the basis of CIDR format since the number behind the / shows how many contiguous bits are set to 1 (one) from the left.255. Let's take the last octet (the last 8-bit) of the /29 CIDR 11111000 The reverse bit is 00000111 As you may notice.11111111.255.11111111.255.255. the last octet will change while the 1st three octets are constant.255.255. You like to know how many /29 subnet will be and how many IP addresses within each /29 subnet. You like to subnet the network into /29 CIDR network. this is how subnet mask looks like OCTET 1st.11111111.192 = 11111111. Say you have a full Class C network. you can see that the 1st 24 bit are set to 1 (one) and the remaining bit are set to 0 (zero) on /24 CIDR. Following is an illustration.255.11000000 255.11111111 From the table.255. this is seen as a reverse correlation between subnet mask and IP address number.255 = 11111111.11111111.255. 3rd .11111111. Let's focus on the last octet.0 = 11111111. Keep in mind that octet is an 8-bit binary.128 = 11111111.11111111.11111111.255. In binary format.255. 11111000 = 1 x 2^7 + 1 x 2^6 + 1 x 2^5 + 1 x 2^4 + 1 x 2^3 + 0 x 2^2 + 0 x 2^1 + 0 x 2^0 = 128 + 64 + 32 + 16 +8 +0 +0 +0 = 248 .11111110 255. 2nd /24: /25: /26: /27: /28: /29: /30: /31: /32: . 4th 255.11111111.00000000 255.11111111.224 = 11111111. As you may realize.11111111.11111111.11111100 255. Reverse Bit Correlation Between Subnet Mask and Number of IP Address Let's review the previous table. Quickly Determine Specific Subnet Range Let's review the previous illustration of subnetting /24 into /29 network. When you cannot use Subnet Zero. To find out the IP address quantity within each /29 subnet from the last octet perspective.100. the all zero) for Network ID and reserve another IP address (the last one. * take the number of bits that are set to 1. To find out the subnet mask quantity from the last octet perspective. the all one) for Network Broadcast.168. Network ID) and last (the all one. which is five * take this five as the power of 2 as 2^5 * 2^5 = 32 This 32 represents the quantity of /29 subnet that you will have when you subnet /24 network into /29 network. From the last octet perspective. With the /29 subnet. Note: This subnet mask reverse bit is the foundation of wildcard (inverse subnet mask) understanding.1 for Network ID 1 for Network Broadcast) for hosts when you subnet /24 into /29 network. then you only consider the last octet.00000111 = 0 x 2^7 + 0 x 2^6 + 0 x 2^5 + 0 x 2^4 + 0 x 2^3 + 1 x 2^2 + 1 x 2^1 + 1 x 2^0 =8 The 248 represents the last octet of the /29 subnet mask and the 8 represents the number of IP addresses within the /29 subnet. »Cisco Forum FAQ »Quick and Easy Wildcard (Inverse) Subnet Mask When you deal with broadcast network.0 network. then there are 6 usable IP addresses (8 . This time you like to see what the 23rd subnet looks like or what the 23rd subnet range is when the /24 network is 192. then there are only 30 usable subnets (32 . As mentioned. Note that so far the assumption is that you can use Subnet Zero. the set-toone bits represents the subnet mask and the set-to-zero bits represents the IP address number within the subnet. Network Broadcast) subnets. which are the 1st (the all zero. which is three * take this three as the power of 2 as 2^3 * 2^3 = 8 This 8 represents the quantity of IP address that you will have within each /29 subnet. Check out following FAQ for further info. * take the number of bits that are set to 0. then you must reserve one IP address (the 1st one.1 for Network ID . 4.1 for Network Broadcast) for hosts. Since you are subnetting /24 into /29. the 1st five bits are set to 1. There is a similar situation with the subnet quantity. . 177.168. you now have the subnet range which shows you the 1st and the last IP addresses. there are 8 IP addresses within /29 network.1) part shows that the formula assumes you use Subnet Zero.100. then the n = 23.168.100.100.100. You like to know how the .179.181.100. (23 . you can add that number by (Number of IP addresses within the subnet . Following is the illustration with the 192.168.192. To find out the last octet Network ID number. 192.1) x Number of IP addresses within the subnet Once you have the Network ID number.1) to have the Broadcast ID number (the last number within the last octet).0/24 subnetting into /29 networks.100.Since this is subnetting /24 into longer prefix.176/29 where Network ID IP address = 192.180.255.182 As you may notice. 5. Since you like to know what the 23rd subnet range is.168.168.168. 192.178.100.100.176 .100. Last Octet Network ID number of the n-th subnet = (n .100.183 = 192. 176 + (8 . 192.168.1) = 183 Therefore the 23rd /29 subnet range is 192. 192.192 subnet mask.183 Usable IP addresses = 192.100. At this point. The formula is therefore alterable easily when Subnet Zero is not in use. 192. you only need to focus on the last octet.168. In other words. you should have the 1st and the last number within the last octet. To quickly determine the subnet range is.168.255.176 Network Broadcast IP address = 192.1) x 8 = 176 To find out the last octet Network Broadcast number. the (n .100. you can use the following formula to determine the last octet of the Network ID number (the 1st number within the last octet).168. where the 1st three octet are constant.168.168. Convert Dotted 4-Tuple Subnet Mask Format Into CIDR Format Let's say you have 255. As mentioned. subtract 8 by 6 to have 2.255. In other word. the 255.192.255. In 255.192 into CIDR format. recall that 255. you find that n = 6 Recall the previous discussion that the n = 6 represents the six set-to-zero bits within the last octet. Don't worry! As usual.192 subnet mask.subnet mask looks in CIDR format. Since there are only 8 bits within a single octet. In representing the network design. Therefore 255. As mentioned.255. .192 subnet mask is equal to /26 CIDR. Therefore you need to use binary to convert. * IP Network IP Network is a network that is based on IP addresses and subnets. there are additional 2 set-to-one bits within the last octet as you earlier find out.255. Find out n where 2^n = 64 By simple calculation (and probably some trial and error). Subnetting and Network Design The simpler approach on how to be more familiar with subnetting is probably by doing a network design.255. CIDR format is based on the binary format.255. There will be three network design processes presented here to illustrate subnetting concept and implementation further. the set-toone bit quantity within the last octet are two. When you build up your own table (or memorize it).255. there is always use of minimal binary calculation :) Following are the steps when deals with /24 subnetting: * Determine IP address quantity within the subnet * Find out n where 2^n = IP address quantity within the subnet * Subtract 8 by n * Take the subtraction result as additional set-to-one bits to the 1st 24 bits set to one * Add the total number of bits set to one * This total number represents the CIDR format Let's use an illustration to convert the 255.0 network has the 1st 24 bits set to one where the remaining 8 bits are set to zero.255. Since CIDR format bases on the 1st set-to-one bits.255.255.255. you know there are 64 IP addresses within 255. * Internet Internet is loosely definied as network outside of local and/or private network. From the same discussion. there are terms and definitions used as follow.192 network has the 1st 26 bits set to one (24 + additional 2) where the remaining 6 bits are set to zero.255. From multiple subnet interconnection perspective.0. In later illustration. all host share the same Network ID and Broadcast IP addresses.127 * Different Network Different Network is a network that has a different Network ID and different Broadcast IP addresses than other IP network. Confused about those terms and definitions? Don't worry. Since switch deals only with host connection of the same network.0.168. These hosts reside within the same range of network. there are two networks of 192.168. In other words.0. it can be used as default gateway to reach those outside subnets.* Same Network Same Network is a network that is shared between multiple hosts. you will see that IP network interconnection is not needed when hosts in the same network need to communicate between each other.0. it is seen as Layer-2 network device and does not by default deals with Layer-3 info such as IP address and subnet mask.0/25 that hosts multiple machines with the same Network ID IP address of 192.0.128 and Broadcast IP address of 192. Since router deals with subnet interconnection. :) Network Design #1 Let's say you have the following situation.168.0.0.168. When this Interface has IP address. Router provides this IP network interconnection mechanism. a network of 192.0. * Switch Switch is a network device that connect hosts within the same network. the IP address is seen as an IP address of a host within specific subnet.168. it is seen as Layer-3 network device. you will see that IP network interconnection is needed when hosts in different network need to communicate between each other. This special functionality is to provide interconnection between hosts of multiple subnets.168.128/30 where the first network has Network ID IP address of 192.0.131 * Router Router is pretty much a host with special functionality.168. * (Router) Interface Router Interfaces or simply Interface is a router port that serve as entry and/or exit point to interconnect multiple subnets.168.168. In later illustration. As example. Instead. Since the IP address serves as next hop to reach subnet outside local subnet.0 and Broadcast IP address of 192.127 and the second network has Network ID IP address of 192.0/25 and 192.0 and Broadcast IP address of 192. you will see that this IP address serves as next hop to reach destination subnet or subnet outside local subnet. As example. There are three different departments . Therefore a router is a network device that interconnect multiple subnets for communication purposes between hosts in different subnets. Keep reading and slowly but surely those terms and definitions come in to your understanding naturally. the hosts just use Layer-2 communication mechanism which is the MAC address as described briefly in Network ID and Network Broadcast IP Addresses previous discussion. 168.32/28 (192. By subnetting a larger network into smaller networks with certain host size the network can hold.0. /29 subnet size is 8.168.0. You can then assign the following subnet for each department 1st Department: 192. you are supplied with one router with 4 interfaces and three switches.16/28 (192. Each department has currently 10 hosts total in form of 7 PC.17 .46) Note that this process of determining subnet mask of certain network to fit specific need of certain number of host within the network is called VariableLength Subnet Masking (VLSM).33 . there should be minimal to no waste of IP address use.0.0/24 to support the entire network. and 1 printer. Following is the network setup.0. To interconnect departments and to allow Internet access.0. For 10 hosts.0.192.168.0/28 (192. there are 16 IP addresses. In /28. the network type choice is a broadcast network.168.168. As in any network.192.0.0.168. 2 servers.that need access within each other and the Internet. To setup the network.168.14) 2nd Department: 192. Internet | | Router ||| +--------------------------+ | +-------------------------+ | | | Switch #1 Switch #2 Switch #3 | | | | | | | | | PC | Printer PC | Printer PC | Printer Server Server Server where each switch is dedicated for each department. the smallest subnet size that fit would then be the /28. there must be some (IP) routing in place . there is no point of creating VLSM to reserve IP address use without some kind of mechanism to provide communication between these smaller-size networks. then there are 14 remaining IP addresses for host.1 . the closest subnets for 10 hosts are /29 and /28.168.168. there will be IP routing mechanism for subnet inter-communication. Since there are multiple hosts within the entire organization. then there would be dedicated /28 subnet for one of each. Since there are 10 hosts for each department.192.168.0. which won't fit 10 hosts. In other words. It is also decided that each department would have independent subnet. Referring to the host IP address availability table. Let's say you are given a Class C subnet of 192.30) 3rd Department: 192.0. Assuming two IP addresses are already reserved as the network ID and broadcast addresses. 168. and 3rd departments.0.16/28.0.1 .0.0/28. Therefore there would be one IP address from each above subnet reserved for the gateway. Therefore the router should handle the gateway IP address. Since each subnet has its own gateway IP address. 1st Department host IP addresses: 192.14/28 2nd Department gateway IP address: 192.for inter-communication between networks or between device in different network such as inter-communication between devices in 1st.168. and Switch #3 is dedicated for 3rd Department. Fortunately.0.168.0. The perfect device to handle the routing is the router.32/28 respectively).168.168.168.1 .168.0.0.168. Switch #2 is dedicated for 2nd Department. this router has 4 different interfaces with routing capabilities. Let's say then 1st Department gateway IP address: 192.240) Gateway: 192. Switch #1: IP address range: 192.168. In routing technology.192. You can then have each router interface to handle gateway IP address of each department and to reach the Internet.46 As mentioned.0.14 Subnet Mask: /28 (or 255.0.255.0. Therefore the following applies to all hosts that connect to the respective switch. The 1st three networks are the 1st.168.17 Switch #3: .168.192.17/28 3rd Department gateway IP address: 192.192.0.255. this gateway device should be able to handle traffic routing between networks.30 3rd Department host IP addresses: 192.168.168.255.18 .38 192. Let's say Switch #1 is dedicated for 1st Department.255.168. Note that there are four networks that need to intercommunicate. 2nd. The last network is the Internet.168.0.0.192. then each subnet requires dedicated router interface to handle gateway business. and 3rd Departments (192.168.0.0.0. there is a network device that act as a gateway to reach outside network. Each of these networks requires dedicated router interface to act as the gateway to handle gateway business of reaching outside network. This gateway in IP network shows as just another host within certain network. In other words.168.240) Gateway: 192. 2nd.168.0. 192.168.0. 192.33 .37/28 Now each subnet has 13 IP addresses available remaining.192.30 Subnet Mask: /28 (or 255. there is only one router for the entire organization network.0.168.17 .36.13 2nd Department host IP addresses: 192. These should be available for all hosts within each department. and 192.0.14 Switch #2: IP address range: 192. 0.192.168.0 is reserved for Network ID * 192.0.168.0.13 range) Subnet Mask: /28 (255.168.168.168.2/28 Here is the detail subnet info IP Address : 192.192.0.0. or to be precise.168.14 IP Address range available : 192.192.0.0.0 Network Broadcast : 192.37 Let's take closer look to 1st Department's hosts.33 .168.168. servers.255.168. printers.0/28 to reach outside network such as the other 192.240) Gateway: 192.168. and any network device within the 1st Department that need network connectivity * When you set network info on (let's say) a server.28/28 IP Address : 192.168.1 .168.168.255.13 range is available for the PCs.0.0.0.168.192.168.0. 192.192. 2nd Department 192.168.15 IP Address range for host : 192.0.255.168.13 The table leads to these understanding * 192. the server network info would be the following IP Address: 192.255.IP address range: 192.46 Subnet Mask: /28 (or 255.240) Number of IP address within the subnet : 16 Number of IP address available for host: 14 .168.14 (pointing to the router) Similar situation applies to other hosts within 2nd and 3rd Departments.0.0.255.0.0.240) Gateway: 192. One host will have the following.255.28 Subnet Mask : /28 (255.2 Subnet Mask : /28 (255.0.0. the router interface that acts as gateway dedicated for hosts within 192.192.0.14 is reserved for gateway (the router.0.0.168.0.15 Network ID : 192.2 (or any IP address within 192.1 .1 .0 .0.0.0.0.255.240) Number of IP address within the subnet : 16 Number of IP address available for host: 14 IP Address range : 192.168.255.168.0.x hosts and the Internet) * 192.15 is reserved for Network Broadcast * 192.168.168.168.168.168.14 Gateway IP Address of the subnet : 192.168.1 . 168.240) Number of IP address within the subnet : 16 Number of IP address available for host: 14 IP Address range : 192.168.192.0.30 3rd Department 192.32/28.168. hosts within all department need to reach other hosts that reside at outside world (i.30 Gateway IP Address of the subnet : 192.47 IP Address range for host : 192.0. there are multiple ways.240 where the router's 1st interface will act as the gateway of the 1st Department's hosts.168. the router then has all of these outside world subnets defined.0.255.240 2nd interface IP Address: 192.168.192.0.168. and 192.0.46 Gateway IP Address of the subnet : 192.168. 192.0. this device .47 Network ID : 192.192.37 Subnet Mask: 255.38 . the gateway should be device that has ability to do routing. To connect to the outside world. 2nd interface will act as the gateway of the 2nd Department's hosts.0. 1st interface IP Address: 192. As mentioned. 192.0.0.168.0.240 3rd interface IP Address: 192.0.0.16/28.0.0.168.0/28.168.168.0.16/28.0.0/28.255. Another way to set the router is to define the gateway.192.168.32 Network Broadcast : 192.0.0.168.17 Subnet Mask: 255.41 Subnet Mask : /28 (255.33 .168.168.255. These outside world hosts do not reside within 192.192.168.17 IP Address range available : 192.168.31 Network ID : 192.0.192.0.168.0.255.192.168.168. and 192. To set the router to have such ability. One way is to define each and every subnet available within the outside world.168.33 .0.168. the router must have ability to reach subnets that are not within 192.IP Address range : 192. As mentioned.0. In addition. the Internet).46 The router will have the following IP address assignment.e.0.168.255.32 .168.0.168.168.17 .31 IP Address range for host : 192.18 .0.168. When the router is running BGP peering with ISP and receive full BGP table.0.168.255.168. 192.16 Network Broadcast : 192.16 .41/28 IP Address : 192.255.255.36.0.168.0.0.32/28. and 3rd interface will act as the gateway of the 3rd Department's hosts respectively.14 Subnet Mask: 255.168.0.37 IP Address range available : 192. 168. Internet and Server Access: Introduction and Practices . In addition. There will be plan to host three new servers that are accessible from the Internet. FYI. The 4th Department host number will be five. Side Note Check out the following FAQ for more info on NAT/PAT »Cisco Forum FAQ »NAT. To provide network security (i. gateway or default gateway term is sometime referred as "the gateway of last resort" since it will be used only to reach unknown or undefined subnets. Port Forward. there are additional two departments. 3rd Department host number remains the same after 2 years.0 [ISP DEVICE IP ADDRESS] The command basically informs the router that to reach subnets that are unknown or undefined.0. To simplify administration. The 5th Department host number will be three.e. PAT. For this connection. The router's 4th interface then will serve the entire organization network connectivity to the outside world. there will be dedicated firewall box facing the Internet. this device IP address is usually mentioned as gateway or default gateway. to protect the Internet-accessible servers).0.0. The perfect choice is the ISP device that the router connects to. Let's call this area "server farm". Some discussions »Break /25 into small and large subnet? »Subnetting question Network Design #2 After some assessment. Should you choose to set gateway to reach outside world. use the ISP device as the next hop. Let's say these two departments are called 4th and 5th Departments.0/24). the command to set gateway is the following ip route 0. you then use the ISP device IP address. the existing servers and printers will be moved and kept under the same area as the new servers. there will be plan to use this firewall box also as NAT/PAT device. it is predicted that 1st Department host number would be triple within 2 years. 2nd Department host number is predicted would be double also within 2 years. the router is utilizing the 4th interface.0. Since you are using Private IP Address (192.0 0. You should assign the router's 4th interface IP address and subnet based on the network info your ISP provides. On the ISP network info.0.must have knowledge on how to reach those outside subnets. In Cisco router. Based on this. 2nd. Internet | | Firewall | | Router 4 | | Switch | | | +----------------+ | +-------------------+ | | | Router 1 Router 2 Router 3 | | | | | | 1st | 3rd | 4th 5th 2nd Switch ||| +---+---+--------------------+-+-+-+-+--------------------+---+---+ | | | | | | | | | Server | Server Server | Server Printer | Printer Server Server Printer Existing Internet-accessible Note that there are multiple network types with this new network design... and Router 4 network. and 3rd Dept. In this case.To support this new setup. and 3rd Dept. Broadcast network type also applies to Router 2 and Server Farm network. you are provided with the following new equipments * * * * One 2-interface firewall box Two 4-interface router One 2-interface router Four switches The following is the new network setup. is a broadcast network. 4th Dept. Router 3.) as Router 1. network. To start. The two new 4-interface routers will be Router 2 and Router 3 respectively. The 2-interface router will then be Router 4. : /27 subnet to cover 30 hosts . Therefore the network type used would be still broadcast network.. 1st Dept. Network of Router 1. and lastly the Router 1. and 5th Dept. only that it consists of two useable IP addresses (somewhat point-to-point network). 1st Dept. 2nd Dept. However network type of Firewall and Router 4 network is point-to-point network. we can keep using the existing router (that serves 1st. Router 2. Router 3. we had hardware and/or media type restriction that is unable to function as point-to-point network. let's recalculate the assigned subnets for each department. 86) 5th Dept. there will be a need to assign specific subnet for specific connection. : /29 subnet to cover 3 hosts Server Farm: /28 subnet to cover 12 hosts We will be minimizing on readdressing all the hosts. Therefore there will be IP address reusing and reassigning.0.86 Subnet Mask: /29 (or 255.192.64/28 (192. : 192. IP address range: 192.255.192.94 Subnet Mask: /29 (or 255.0.168.0.224) Gateway: 192.168.81 5th Dept.192.89 .192.168.168.81 . : 192.33 .168.192.168.255.0.168.0.168.192.255.78 4th Dept.168. IP address range: 192.0.168.168.168.81 . Firewall.168.240) Gateway: 192.0.1 .30) 2nd Dept.62) 3rd Dept.0.14 2nd Dept.192.0.192.192.168.240) Gateway: 192. : /28 subnet to cover the same 10 hosts 4th Dept.0.224) Gateway: 192.255.255. : 192.33 .168.110) Network Info 1st Dept.0.168. IP address range: 192.168.168.0. : 192.168.255. : /27 subnet to cover 20 hosts 3rd Dept.0.62 Subnet Mask: /27 (or 255. IP address range: 192.30 Subnet Mask: /27 (or 255. and the Internet.0. : /29 subnet to cover 5 hosts 5th Dept.255.168.0.110 Since there will be routing between four routers.255.168.0.255.1 .168.168.0.248) Gateway: 192.168.0.97 .0/27 (192.0.0.168.168.0.168.0.0.33 3rd Dept.168.94 Server Farm IP address range: 192.88/29 (192.255.110 Subnet Mask: /28 (or 255.0.65 . Between Router 4 and Firewall Host # :2 .168.0.168.168.168.32/27 (192.168.0.255.89 .2nd Dept.192.96/28 (192.78 Subnet Mask: /28 (or 255.0. 1st Dept.192.168.0.0. IP address range: 192.97 .0.0.0. : 192.0.94) Server Farm: 192.168.248) Gateway: 192.80/29 (192.0.65 .0.255.78) 4th Dept.192.0.168. 255.0.Smallest Subnet : /30 Subnet Assignment: 192.250 Subnet Mask: 255.248 To route between routers.255.14 Subnet Mask: 255.255. To support these routes.255.33 Subnet Mask: 255.0.0.0.168.168.224 2nd interface IP Address: 192.248 Router 2 4th interface .255.168.168.248 2nd interface IP Address: 192.0.255.168.224 3rd interface IP Address: 192.255.110 Subnet Mask: 255.0.248/29 Router 1 1st interface IP Address: 192.168.168.0.168. there will be a need to assign specific IP address to specific router interface.168.255.94 Subnet Mask: 255.0. Router 1 4th interface IP address: 192.255.240 Router 3 1st interface IP Address: 192. there must be routes of reaching specific subnets using specific next hop IP address.240 Router 2 1st interface IP Address: 192.255.240/30 Between Routers Host # :4 Smallest Subnet : /29 Subnet Assignment: 192.0.255.78 Subnet Mask: 255.81 Subnet Mask: 255.255.255.255. 249 as as as as the the the the next hop to reach 192.168.168.168.168.0. you can supernet both /27 networks to be a /26 network.80/28 * Use 192.0.0.0.255.0.0.64/28 next hop to reach 192. Router 1 is presenting two consecutive /27 networks.168.168. To support these routes. Lastly 192.249 as as as as the the the the next hop to reach 192.168.96/28 gateway To route between Firewall and Routers.250 192.0.0/26.0.168.252 Subnet Mask: 255. .80/28.168.248 Router 4 1st interface IP address: 192.251 192. followings are the route reachability * Use 192.IP address: 192.0.0.80/28 (192.0. followings are the route reachability * * * * Use Use Use Use 192.0.168.64/28 subnets are under Router 1.255.0/26 (192.94) subnet is under Router 3.0.168.168.250 192.168.192.0.168.0.255. In other words.0.0.0.255.0.0. To conserve router's resources.168.0.168.168.168.0.0/26 next hop to reach 192. From Router 1 perspective. 4th and 5th Depts subnets (that the Router 3 presenting to other routers) are also candidates for supernetting of 192.168.168.255. followings are the route reachability * * * * Use Use Use Use 192.0/27 and 192.250 192.248 Notice that 192.0.252 as the next hop to reach 192.62) and 192. there must be routes of reaching specific subnets using specific next hop IP address.168.250 192.252 192.168.168.64/28 next hop to reach 192.0.168. Similarly.168. You then have a choice to combine these two /27 networks to become a single larger /26 network of 192. From other routers' perspective.0. we then supernet the networks. 192.168.0.96/28 * Use 192.249 Subnet Mask: 255.0.168.248 Router 3 4th interface IP address: 192. there will be a need to assign specific IP address to specific device interface.0.168.168.96/28 is under Router 2.0.255.168.249 as the gateway From Router 2 perspective.168.0/26.80/28 gateway From Router 3 perspective.81 192. the 192.168. Let's take close look on the 192.0.168.168.1 .32/27.0. Similarly.0.251 as the next hop to reach 192.0.0.251 Subnet Mask: 255.0/26 next hop to reach 192.168.0. 0.249 Router 2 ip ip ip ip route route route route 192.0.96/28 From Firewall perspective.168.168.0.255.0.168.0.0. Router 1 ip route 192.251 ip route 192.168.255.240 192.251 192.0.255.168.255.168. followings are the route reachability * Use 192.255.248 192.0.0.255.0 192.250 192.0.0.0.0.0 0.255.64 255.0.168.168.168.168.0.0.252 0.168.0.255.168.0 255.252 Firewall 1st interface IP address: 192.0 255.255.168.255.0.0/26 192.0.0 192.224 192.255.255.0.96 255.168.240 192.0.0.255.168.0.0.168.168.0.168.168.0.168.0.250 192.255.255.249 Router 3 ip ip ip ip route route route route 192.0.0.168.0.64/28 192.0.168.0.0.0.255.64 255.242 as the next hop to reach 192.168.0/24 (another supernet) * Use ISP Device IP Address as the gateway With this new network design.80 255.168.255.168.0 192.0.250 192.0 0.255.0.96 255.255.250 192.242 Subnet Mask: 255.168.0.168.0.248 192.0.0.241 Subnet Mask: 255.80 255.252 192.0.0.251 0.0 0.168.241 as as as as as the the the the the next hop next hop next hop next hop gateway to to to to reach reach reach reach 192.0.168.224 192.255.168. similar route statements are in place as follow.Router 4 2nd interface IP address: 192.249 . followings are the route reachability * * * * * Use Use Use Use Use 192.250 192.252 2nd interface IP address: refer to the ISP network info Subnet Mask: refer to the ISP network info From Router 4 perspective.80/28 192.168.252 ip route 0.240 192.240 192.250 192.0.168. the ip-route statement is a command in Cisco routers to setup static route in IP network.168.224 192.0. One group will be the Internet accessible server group and another group will be the non-Internet accessible server group.0.0.251 0.168.0 [ISP DEVICE IP ADDRESS] As a note. it is a good idea to separate them from the existing servers by group.168.168. INTERNET | | Internet Router | | Router 1 ------.255.0.0.0.250 192.168.255.Router 4 ip ip ip ip ip route route route route route 192.80 255.252 192.0.241 Firewall ip route 192.0/24 to use for private .168.Switch 2 Switch 3 ---.0 192.168.255. This separation objective is to provide more secured network and broadcast traffic control.64 255.250 192.0.255. Let's have another look of network design.255.168. The separation involves assigning dedicated subnet to each server group.Switch 1 --.255.0 192.168.255. you are assigned 192. Network Design #3 You may notice that above network design is pretty much what small business have as their network.Router 3 | | | | LAN 2 LAN 3 As previous illustration.96 255.255.0.0. creating specific route statements as defined above.0.255.0.96/28 network or reassign/renumber network devices with minimal changes on the existing network design.168. Suggestion Since only the new servers are Internet accessible.0.0 0.0.0 255.0 255.242 ip route 0.168.0.0.168.0.0.0.0.240 192.LAN 1 | | | | +--------------------------------+ +-----------------------------------+ | | | | Router 2 ---.240 192.0 0.248 192.255.168. You should be able to do the work by either subnet the existing 192.0. This time you need to setup private network that consist of three sites as follows. where .194 is for Internet Router LAN 1: 192. where .160/27. check out the following FAQ. Main Office tends to house more hosts to serve all offices. this network design could be a hub-and-spoke private network where the Router 1 and LAN 1 is the Main Office where others are Branch Offices.196/30.168.161 is for Router 3 (also LAN 3 default gateway) and .0. Therefore you may need to assign larger subnet for Main Office and to assign smaller subnet for Branch Office. This network design could also be a local network within one office where the Router 1 and LAN 1 is the Main router with its network and each of other routers represent departement router.168. where .0.158 are for hosts Site 3 Router 1 .0.129 is for Router 2 (also LAN 2 default gateway) and .191 are for hosts Following are the network setup with their associated subnet assignment INTERNET | | Internet Router . Router 1-Internet Router * One /25 subnet for LAN 1 * Two /27 subnets for other LAN: LAN 2 and LAN 3 Side Note: In real-live network.168. * Three /30 subnets for point-to-point network: Router 1-2.0. where .168. For more sample network design.201 is for Router 1 and .Router 3: 192. where .. In addition.168.168.0.1 is for Router 1 (also LAN 1 default gateway) and .168.168.0.126 are for hosts Site 2 Router 1 .196/30..0.128/27.162 . Following is one way of design the network. 192.network.202 is for Router 3 LAN 3: 192.0.Router 2: 192.168. where .128/27.198 is for Router 2 LAN 2: 192.193 is for Router 1 and .Internet Router: 192.192/30.200/30 One /25 subnet : 192.2 . 192..168.0. 192. Router 1-3.160/27 Site 1 Router 1 .193 | .0.0.130 .0.192/30.168. »Cisco Forum FAQ »Should I use Layer-3 switch or router? Subnet Assignment Three /30 subnets: 192.197 is for Router 1 and .0/25.0/25 Two /26 subnets : 192.168.200/30. 0.1 .0.0.128/27): through 192.0.224 192.Switch 1 --.Switch 2 Switch 3 -----------------.0.160/27): through 192.168.0.161 | | LAN 2 LAN 3 .197 Router 3 To reach anyplace (other LAN and the Internet which is in other words 0.2 to .160 255.0..129 | | .0.0.168.0.0.0.0.0.168.168.0.0 0.168.0.128 255.Router 3 .202 Router 2 To reach anyplace (other LAN and the Internet which is in other words 0.LAN 1 | | .168.0/25 Router 1 ---------------.0/0): through 192.255.168.168.168.0.198 192.0.200/30 | | +--------------------------------+ +-----------------------------------+ | | .168.168.168.0.198 name LAN 2 ip route 192.0.0/0): through 192.| 192.0.202 Router 2 -----------------.0.0.255.0.194 | | 192.202 name LAN 3 Router 2: ip route 0.0.191 IP Routing Reachability Router 1 To reach the Internet (0.201 Internet Router To reach the Internet (0.0.197 | | .168. 126 | | .0.168.0.168.0.0.0/0): through 192.168.168.255.193 To reach LAN 2 (192.0.162 .193 name Internet Access ip route 192.158 .0.0.196/30 | | 192.255.0.0 192.130 .192/30 | | .0.0 192.168.197 name Outside Access Router 3: ip route 0.201 name Outside Access Internet Router: .0.160/27 | .0.168..168.0.0 0.168.0.128/27 192.0.168.0/24): through 192.224 192.0.0 0.0.201 192.0/0): through ISP router IP address (or the Internet Router default gateway) To reach any private sites (192.194 Routing Statement Router 1: ip route 0.0 192.198 To reach LAN 3 (192. 0.168.0.168.1 192.0 192. this 192.1 192.0. Therefore the traffic will be forwarded via the 192.168.0. .168.0/24 range.168.160/27 route.160/27 subnet.0.168.0. This IP address falls within the 192. Second Illustration Now let's say there are following routes ip ip ip ip ip route route route route route 0.0. This IP address does not fall within the 192.168.193 192.164 255.224 192.0.202 Let's say there is a traffic that need to reach a host with IP address of 192.168.128 255.165.0.0.0.0.168. CIDR Longer and Shorter Prefixes in Routing As shown on previous network design.255.160/27 routes.128/27 nor the 192.0.0.0 255.0 0. This IP address falls within the 192. This IP address does not fall within the 192. There is however a route to reach any IP address that falls within 192.202 IP address host is called the next hop IP address to reach the 192. When there are no available routes to reach specific subnets. First Illustration Let's review the following routes ip ip ip ip route route route route 0.255.224 192.168.0.168.255.230 Let's review the previous illustration that say there is a traffic that need to reach a host with IP address of 192.0 [ISP DEFAULT GATEWAY IP ADDRESS] ip route 192.0.160/27 route.168.0.168.0.168. There is however more specific route to reach a host with IP address that falls within the 192.168.0.168.255. then the default gateway route is chosen as the last resort.16.0.0.255.168.0 255.168.224 192.0.128 255.ip route 0.0.168. let's say there is a traffic that need to reach a host with IP address of 192.128/27 nor the 192.255.255.0.0.16.0. This IP address does not fall within the 192.168.2.0 0.160/27 routes.0.255.198 192.168.0.0.193 192.0.255. 1. 2.255.0.0.194 name Private Networks Classless.168. Therefore the traffic will be forwarded via the 192.0. You may notice that these routes to reach specific subnets are preferable to reach the intended subnets or IP addresses.168.0 0.168.0. let's say there is a traffic that need to reach a host with IP address of 192.168.255.0.168.0.168.255.168. Therefore the traffic will be forwarded via the 192.198 192.255.0.0.1 host.0/24 range either.168.0.0 192.0.224 192.168.0.255.0 192.168.0.0 255.255.168.165. Classful.0 192.0.0. There is however a route to reach any IP address that can be used as last resort when there is no match to any of the specific route available. As a note.0 192.168.168.0.160 255.202 192.164/30 route.0. For the next example.160 255. As a next example.168.255.168.0.252 192. there is a default gateway route and there are routes to reach specific subnets.202 host.193 host. 0 0.168.0.0/8.0. Similarly. B.0/16.168.0.0/24.168.0.0. there are terms called Class A.168.0.0.0. With the need of reaching a host with IP address of 192.160/27 route.168. 172.0. let's review the following routes ip route 0.1 host instead of 192.255.230 host instead of 192.0.198 ip route 192.165.128 255.0/24. following is a list of some Class A. Instead the traffic will consider the 192.1 ip route 192.168.0/24 Let's say there is a route that fall under 10.0.165 will be forwarded via the 192.0.0/24 are ignored. 3.0/24.43.255.0.255. 192. In the case of reaching IP address that fall under 192.0/24 or will take the default gateway route as the last resort when available. Therefore the traffic that need to reach a host with IP address of 192. any specific routes than 192.168.202 ! no ip classless Let's say there is a traffic that need to reach a host with IP address of 192. the other two routes with more specific routes or have longer prefix than 192. route with more specific info or in other words longer prefix is preferable to the less specific info or in other words shorter prefix. This statement means that any CIDR routes with more specific than their classful route are ignored.168. This IP address falls within the 192.0.224 192.168.0.168. any routes that are more specific or have longer prefix than the 192.0/16 Class C network: 192. traffic will take either the full class network route or will take the default gateway route as the last resort if available.0.1.17.255. B.16.168.0/8 Class B network: 172.0.0. There is however a statement of no ip classless is being used.168.255.0.193 ip route 192.0.168.0.0.0.202 host. When the no ip classless is being used.165.0.Note that in IP routing.168.165 will be forwarded via the 192.168.168.168.168. As example. When more specific routes that have longer prefix are being ignored. and C network available in IP (IP version 4) network.0.0.0. Third Illustration As a next illustration.168.0.0. and C network.0.0/8 to reach IP address that fall under 10.168.168.0.0. 192.0/8 will be ignored.255.0 255.168. Class A network: 10. Therefore the traffic that need to reach a host with IP address of 192.0.168.0/24 will be ignored and instead the traffic will take the route to reach the entire 192.168.160 255.0.168.0/24 route.0.0 192.0. any specific routes than 10.202 host.0.224 192. On early discussion.0 192.0.0/24 will be ignored when the no ip classless is being used. In the case of following routes . This statement also means that a classful routing is used and will ignore any subnetted network.168.0/24 to reach IP address that fall under 192. 0/25) where C is within different network (192. You can check out the following for more info on Classful and Classless routing system Wikipedia . By checking IP addresses and Subnet Masks of itself and the other host's. any routes that have longer prefix than their associated Class A.168.0. The network is sometime called Broadcast Domain.0. Let's review the following illustration.168.0. more specific routes or routes with longer prefix are preferred.168.0/24 route. There is also an IP address of the host used as gateway to reach the intended host IP address. the hosts are called within the same network or within the same broadcast domain.255. and C with IP address of 192. From the subnet mask.0. In Classful routing system (as indicated by the no-ipclassless statement). Subnet Mask. .Classful network TCP/IP Guide .193 since there is no specific route to reach the 192. 192.168. Let's discuss this broadcast domain further.224 192.0.255.165 will be forwarded via the 192. and Broadcast Domain As mentioned earlier.56/25.224 192. or C network as result of subnetting are ignored.0.0 0.130/25 respectively.193 ip route 192. B and C Network and Host Capacities Some discussion »[CCNA] Undertanding Next hop routing answer IP Route. As you may note in this illustration. B.0. There was a mention of a Layer-2 mechanism called Broadcast used for communication between hosts within the same network. The Routing Table holds the IP route availability in form of Network ID with its associated Subnet Mask.168.ip route 0.0 192.0.0.0.0.0.168.168. B. a Classful routing system is used instead of a Classless routing system.168.0.168.255.0.198 ip route 192.128/25). With Classful routing system.160 255. This IP route availability is called IP Routing Table or Routing Table in short.0.255.0. since the broadcast traffic only exists within the network (within the domain) and does not by default leave outside the network.202 ! no ip classless then a traffic that need to reach a host with IP address of 192. There are three hosts A. a host can tell if the other host in communicating partnership is within the same network or is outside network.168. and 192.168.168.Class A.128 255. you can see that both A and B are within the same network (192.10/25. the traffic flow is determined based on IP route availability. This IP address is called the next hop IP address. When there are hosts within the same network.0. You may note that this Broadcast Domain relates closely to Subnet Mask.168. However host C also thinks that host D is within the same network as C itself since /24 subnet mask includes the 192. Now let's say there is a host D of 192. B. Both hosts A and B simply do their Layer-2 mechanism without the necessity of routing the traffic flow. Similarly by using the mechanism.168.10 with /25 subnet mask.168. Host B can tell that B is within the same Broadcast Domain as A. C knows that A is at outside the Broadcast Domain of C. The same consequences take place when B needs to communicate with C. and C). the same mechanism of checking IP address and subnet mask takes place.168. this confusion create routing and traffic flow ambiguity which lead to inability to communicate between host D and the rest of the network (say A. B. D sees A.56 with /25 subnet mask. Similarly by checking IP addresses and subnet mask. Since both hosts A and C know that they are at different network. A knows that C is at outside network (outside the Broadcast Domain) of A. . The existance of such host D as you may note create confusion.6/24 IP address.0. and B's IP address and subnet mask.As example.0.0.0. When A needs to communicate with C of 192. Host A needs to communicate with Host B of 192. Host A can tell that A is within the same network (the same Broadcast Domain) as host B.130/25 let's say. From the IP address. both hosts know that there will be no route necessary to communicate between each other. By checking its own IP address and subnet mask. In real network.168.0. both hosts know a Layer-3 IP routing mechanism is needed. the host A has IP address of 192.128/25. and C as hosts that stay within the same subnet as D itself due to the host D's subnet mask identity. hosts A and B think that the host D is within the same network as them. Since both hosts A and B know that they are in the same Broadcast Domain. Using the mechanism.168. From host D perspective.