Understanding Variable Length Subnet Masks (VLSMs) Part 1

At the most basic level, VLSM represents a hierarchical network addressing scheme. Think of it as subnetting multiple times within the same address range. For example, we can take the network and subnet it into a number of large subnets. In our case, these will be used to define large geographic regions. Next, we look at those regions (or large subnets) individually, subnetting them further. This allows us to break up a region for the purpose of defining custom subnets that meet our size requirements. This additional subnetting can continue until you run out of subnets within a given range. A properly designed hierarchical addressing scheme both makes better use of the address space, and also provides for more optimized routing tables, as we’ll see shortly. This figure outlines the basic idea behind a hierarchical addressing arrangement. Note that I have only shown the breakdown of a single large subnet rather than both large subnets, in order to keep things clear.

Using VLSM addressing on a network is slightly more involved that using the same subnet mask throughout. You again need to properly characterize your network, defining how many WAN links, subnets, and hosts per subnet you are dealing with. The easiest way for us to begin is with a single network address. In this case we’ll use the private address I’m going to assume that our company has four main geographic regions, and that each region includes many smaller offices.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of 2000Trainers.com. He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.