Up till now, we have seen Classful subnetting where we can determine the subnet mask of based on the class of the IP address. This is an easy way of finding the subnet mask and determine the network and host bits of the IP address. However, if you have a huge network and a huge number of endpoints, it might be a good approach customize your IP scheme to cater for your specific network and hosts requirement.
Why do we need classless subnetting or Variable Length Subnet Masks (VLSM)
If you are an administrator of a huge network and your current IP scheme is for example, a class A network (10.0.0.0). There you have 8 network bits and 24 host bits.
What if you want to support a large number of networks or subnets within the same IP range. That’s where you will need Variable length subnet masks.
For example, here’s how your current IP scheme may look like with VLSM:
10.0.1.0/24
10.0.8.0/26
So you can a different subnet mask within the same class A network. This will allow you to support a greater number of subnets and hosts.
Here’s the formula for determining the number of subnets your network can support and the number of hosts.
Number of Subnets = 2 ^ subnet bits
Number of Hosts = 2 ^ host bits – 2
Example 1:
Here we have a Class A IP address and we are going to use a variable length subnet mask to divide the network and support multiple subnets.
As you can see, this network can support 65,356 subnets and 254 hosts in each subnet. This kind of network can be a ideal match for a big enterprise that has multiple offices in different regions and they want to support a good number of sub networks.
Example 2:
Here we have a Class C IP address and we want to support multiple subnets within the same network.
This network will support 4 sub networks or 4 subnets. And each subnet will support 62 hosts. This is a perfect example for a company that is small in size, has limited requirement for subnets and hosts.
