Understanding Variable Length Subnet Masks (VLSMs) Part 2

Let’s start off by breaking our network into 6 large subnets, since using the required 4 would only account for current regions, and would not allow for future growth. Doing this is not in any way different that what we’ve done in the past. To get 6 large subnets, we’ll need to steal 3 bits, since 23-2 gives us 6 subnets. In this case, our new mask becomes, or /19. Each of the new subnet IDs is listed in this table, including the location that it will be used for.

Since we’ve used a /19 subnet mask, each of these ranges supports up to 213-2 or 8190 hosts as things presently stand. However, we’re not done. Our next goal is to take one of the areas and further divide it into a number of smaller networks. For example, let’s say that North America includes 16 offices. Of these, 3 are large, and need to support up to 400 hosts. 13 are smaller, and need to support 150 hosts maximum. In the future, it is possible that more large and small offices will be added.

In order to deal with these requirements, we need to subnet the North America network further. The easiest way to deal with this is to consider to be just another network ID, and our starting point. To begin, we know that we need to immediately support 3 large offices, each with up to 400 hosts. For this example, I’m going to have us define one additional large office, in order to account for future growth. That gives us a requirement of 4 networks, and 400 hosts per network. Using the host portion, we would need 9 host bits in order to support our requirement, which would give us 29-2 or 510 hosts per subnet. In doing so, we are left with 4 bits to define subnets, as shown. Remember that our starting subnet mask is or /19 in this case.

Using the first 4 available ranges to define our four large LANs, we would allocate the subnet IDs as shown in this table.

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.