Understanding Cisco Subnetting

IP Subnetting

In late 1980s, IP subnetting was introduced as an extension to the original scheme. Engineers gradually realized that the original class-based IP addressing design was too inflexible for many applications.

What is Subnetting?

There could be more than one definition for subnetting but the best definition of subnetting in which subnetting explained more better is that by default a network id has only one broadcast domain. Subnetting is a process of segmentation of a network id into multiple broadcast domains.

Subnetting is originally referred to the subdivision of a class-based network into many subnetworks, but knows it refers generally to the subdivision of a CIDR block in to smaller CIDR blocks. Subnetting allows single routing entries to refer either to the larger block or to its individual constituents. This permits a single routing entry to be used though most of the Internet, more specific routes only being required for routers in the subnetted block.

Network Subnetting

In a subnetted network, there is an extended network portion. For example, a subnet mask of 255.255.255.0 would subnet a class B IP address space using its third byte. Using this scheme, the first two octets of an IP address would identify the class B network, the next octet would identify the subnet within that network, and the last octet would select an individual host. Since subnet masks are used or bit-by-bit bases, masks like 255.255.224.0 (three bits of subnet and thirteen bits of host) are perfectly normal.

There are several restrictions applied in a traditional subnetted network. Many of these restrictions have been lifted by CIDR, VLSM and more flexible IP routing protocols such as EIGRP and OSPF. However, if other routing protocols such as IGRP and RIP are used, the two restrictions must still be observed are as follow:

• All subnet masks must be of a fix length. Since IGRP and RIP routing updates do not include subnet mask information, a router must assume that the subnet mask with which it has been configured is valid for all subnets. Therefore, a single mask must be used for all the subnets of a given classful network and different subnet masks can be used for different classful network addresses. This rule is referred to as the rule of FLSM (Fixed Length Subnet Mask). Based on the assumption of FLSM, router can exchange subnet route with other routers within the network. Since the subnet masks are identical across the network, the routers will interpret these routes in the same manner. However, routers not attached to the subnetted network can not interpret these subnet routes, since they lack the subnet mask. Therefore, subnet route are not relayed to router on other networks. This leads to second restriction.
• A subnetted network can not be split into isolated portions. All the subnets must be contiguous, since subnet routing information can not be passed to non-members. All the subnets must be able to reach all other subnets with in a network without passing traffic through other networks.

Class C Subnetting

The Class C subnetting is less complicated then the other two classes of IP Addresses. There is comparatively less calculations you have to do in this type of subnetting. For example your company is using a single class C network of 192.168.0.0 with a default subnet mask of 255.255.255.0. The company has six departments of 30 hosts each and the requirement of your company is to segment them and broke the single broadcast domain for security reasons and to increase the maximum availability of bandwidth. You have to do three bits of subnetting using the formula 2n -2 where n is the value of subnet bits. The subnet bits would change the host portion of the subnet mask which is now 255.255.255.224 after subnetting. This can also be written in bit count format such as 192.168.0.0/27.

A common subnetwork environment is too inflexible when you require various types of subnet mask for the same network address. For example, consider a large organization with a single class C address of 192.168.0.0. Its headquarters site is made up of one subnet with 120 hosts on this subnet. The same organization has three regional offices, with a single LAN with less then 30 hosts each. Finally, this organization has six field offices. Each field office has a single segment with less then five hosts each. Which of the following subnet mask is best for this organization?

• A 25 bit subnet mask yielding 2 subnets with each subnet yields 128 valid host addresses each (255.255.255.128).
• A 26 bit subnet mask yielding 4 subnets with each subnet yields 62 valid host addresses each (255.255.255.192).
• A 29 bit subnet mask yielding 30 subnets with each subnet yields 6 valid host addresses each (255.255.255.248).

The answer is the 25 bit subnet mask can be deployed at then central site. The 26 bit subnet mask can be deployed at the branch offices while the 29 bit mask can be deployed at the field offices. This is an example of a Variable Length Subnet Mask. However, the FLSM environment can not accommodate deploying all of these different length subnet masks for a single classful network prefix.

There are two types of subnetted environment such as Fixed Length Subnet Mask (FLSM) and Variable Length Subnet Mask (VLSM). The selection of routing protocol also determines whether you are stuck with a FLSM environment or whether you can deploy VLSM.

Subnetting Calculators

Subnetting calculator is a very useful tool that is available on subnetting tutorials offered by various websites. We can utilize it by downloading from any website for subnetting practice. The subnetting of Class A and B Addresses is comparatively more difficult and complicated then Class C Address. Subnet calculator makes it easier.

Related Articles

1. Understanding Subnetting
2. Subnetting
3. Understanding Variable Length Subnet Masking
4. Understanding Subnet Mask
5. Understanding Cisco IOS