Understanding Variable Length Subnet Masks (VLSMs) Part 3

This article is the final installment in a series of tutorials about VLSM. It all begins here.

Recall that from this table, many IP address ranges are still available on our network – everything from through for North America. For that reason, we can again subnet the address range further to account for the smaller offices. For these smaller offices, we’ll use a /24 subnet mask – this meets our requirement of supporting up to 150 hosts on each of the small LANs. In this case, we can support 254 (28) hosts on each. This table outlines the 13 subnets for the small offices, along with a few defined for additional growth.

Notice that even after dividing up the address space for our small offices, we still have additional address space left over – in this cases, everything from up to is still unassigned. This space could be used to assign additional large or small subnets in North America, as need dictates. However, we still have one last requirement – WAN links.

Remember that a point-to-point WAN link still counts as a subnet, even if it is limited to only 2 hosts. In order to account for our WAN links, let’s use the use the range, and subnet it further. In this way, we have left the entire address space between and for defining additional North America LANs if required.

Since we only require 2 addresses for each WAN link, we only need 2 host bits – 22-2 is 2, which meets our requirements. If we make all the remaining bits subnet IDs, we can support up to 26-2 or 62 WAN subnets within North America alone. This will easily meet our needs. Based on this scenario, our WAN links will use a subnet mask of This table outlines only the first 4 WAN links, since listing all 62 would serve little purpose.

Obviously VLSM gives us better control of the size of our subnets, allowing us to allocate addresses in our chosen space more efficiently. But what other benefits does this scenario bring? Well, one is certainly smaller routing tables. For example, all networks in North America can now be reached from any other geographic region via a single routing table entry – Once any data for North America is sent to this router, it then decides where the data needs to be forwarded next. If we weren’t using CIDR and VLSM in this example, each router’s routing table would need entries for every network in North America. By aggregating all the entries behind a single entry, routing performance will be greatly increased – especially on very large networks.

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.