Cisco CCNA

Subnetting with Variable Length Subnet Masks

With Variable Length Subnet Masks (VLSMs), you carve an address space (such as a major net) with masks of varying lengths to design subnets of different sizes. This allows you to deploy subnets that are appropriate in size to the number of hosts you need to support in a given part of the network. As a result, you can gain efficient consumption of your address space and— depending on how you deploy the addresses—flexibility in the future as you adjust the size of each subnet to handle growth.

NOTE Your routers must be running a routing protocol that supports VLSM, such as OSPF or EIGRP. RIP and IGRP are classful routing protocols and do not support VLSM. Classful routing protocols are limited to a single subnet mask per major net.

Here is the basic technique for variably subnetting a major net:

1 Subnet the space (for example, a major net) into large address blocks based on the large subnets you need in your network.

2 Deploy these large blocks of addresses to support your large subnets.

3 Take any unused large blocks and subnet them further to support smaller subnets with fewer hosts. You can think of this as a second round of subnetting.

4 Deploy the subnets from the second round of subnetting.

5 With additional rounds of subnetting, continue dividing unused blocks of addresses into multiple smaller subnets and deploying them as needed.

Some binary is involved here. Subnetting requires that you understand and visualize binary patterns and apply those patterns to masks. Consider the following example that uses a class C major net.