Variable Length Subnet Masking

Using VLSM

Look at the following network:

• 192.168.1.0/24 = 1 network with 254 hosts

While this may work fine, what if your network requires more than one subnet? What if your subnets have less than 254 hosts in them? Either situation requires some changes to be made. If you applied a /26 mask to your network instead, you would get this:

• 192.168.1.0/26 = 4 subnets with 62 hosts

If that wasn’t suitable, what about a /28 mask?

• 192.168.1.0/28 = 16 subnets with 14 hosts

You can refer back to the Subnetting Secrets cheat sheet design section to help you work out how to apply VLSM to your network or to an exam question. With the /26 mask, you can see how many subnets and hosts you will get:

You have to take away 2 bits for the hosts, so you get four subnets, each with 62 hosts.

Slicing Down Networks

The point of VLSM is to take your network block and make it work for your particular network needs. Taking the typical network address of 192.168.1.0/24, with VLSM, you can use a /26 mask and now do this:

This may work fine until you realise that you have two smaller networks on your infrastructure which require 30 hosts each. What if three of your smaller subnets are taken (marked as IN USE above) and you have only one left (i.e., 192.168.1.0)? VLSM lets you take any of your chopped down subnets and chop them down even further. The only rule is that any IP address can be used only once, no matter which mask it has.

If you use the design section of the Subnetting Secrets cheat sheet, you will see which mask gives you 30 hosts:

The upper section of the chart (not shown here) tells us that three ticks down in the left column gives you a mask of 224 or /27 (3 stolen bits).

You can’t use the .64 subnet because this is already in use. You are now free to use either of the other two subnets. If you needed only one, you could chop down the remaining one to give you more subnets, each with fewer hosts.

Please also read bonus stuff on VLSM on the in60days.com website.