VLSM – A Very Brief History

You read in the subnetting section that we were about to run out of IP addresses.  One way to preserve IP addresses was to allocate certain ranges of IP address which can be used for free on your network so long as you don’t use them to get out to the internet.  If you want to get out to the internet using them then your router can just swap it for a routable address using a technique called Network Address Translation (NAT).


Free addresses to use are: – anything starting with a 10 to – inclusive – any address starting with these two numbers


Using VLSM

Say you are using the following number: which if you have read the subnetting section you know lets you use 254 hosts on your network.  If you only have 20 hosts on your network it means you have wasted 204 addresses.  Now imagine if you are paying for each block of addresses, you are going to waste a huge amount of money as well.


VLSM lets you cut your network into smaller chunks which are called subnets.  All you have to do it add some numbers to the end of your subnet mask and your network will see that you are subnetting it down.  Please see the easy way to subnet section for more information on how to actually do it.


How VLSM Works

If you change your subnet mask from to your network device will see the change and let you break down your network into smaller parts.  That actual number tells the network to chunk your network down into subnet blocks of 32.  If you use the subnetting cheat chart you will see how this actually works.


The subnet mask gives you 8 subnets with 32 available addresses.  Each subnet must start with a subnet number and end with a broadcast number so technically that leaves you with 30 host addresses per subnet.  This is still plenty if you only needed 20 per subnet.

We have gone from one subnet with 255 addresses:

















To 8 subnets with 32 IP addresses per subnet.



















A RFC (request for comment) was submitted which suggested that we could allocate numbers to hosts on networks instead of hostnames.  The current implementation of this scheme is IP version 4 although IPv6 is now replacing this.


The idea is that every address would be made up from four groups of eight binary numbers.  Each group of eight binary numbers is known as an octet.  Because we struggle to write numbers out in binary we usually convert them into decimal but computers and network devices still see all numbers in binary because they can only recognise on and off signals so either a 0 or a 1.


If you want to see how to apply this to a network then please read the easy way to subnet.  There is a lot more in Subnetting Secrets.  More than we could fit on one web site in fact.