The previous article served as a good introduction to Supernetting (CIDR). We analyzed the Supernetting concept and how it helps bind multiple networks into one, decreasing the size of routing tables and saving valuable memory and CPU cycles on routers. This article take a step further by analyzing a Supernet example down to the bit level and covering best Supernet practices.
NOTE:This page requires you to have basic knowledge and understanding on Internet Protocol, Subnetting and Binary notation. These are covered in great detail on other pages and I recommend you have a quick look over these topics if you think you're not up to scratch.
Guideline - Rule to Supernetting / CIDR
Before we get in to deep waters, we must talk about the main rule that applies to creating Supernets. For our example, this rule dictates that, in order to create Supernets from Class C IP Addresses, the network address must be consecutive and the third octet of the first IP Address must be divisible by two.
If we had 8 networks we wanted to combine, then the third octet of the first IP address would need to be divisible by eight and not two.
There is one more rule you should know and this rule has to do with the routers of the network, which will need to work with the new changes. This rule dictates that all routers on the network must be running static routing or using a classless routing protocol such as RIP2 or OSPF. Classless routing protocols include the subnet mask information and can also pass supernetting information. Routing protocols such as RIP1 do NOT include subnet mask information and would just create problems!
Here is an example involving two companies that want to use Supernetting to solve their network requirements. We are going to determine which company mets the criteria for a Supernet (we are assuming the routers are setup in a way that will support supernetting):
As you can see, Company No.1's network passes the test, therefore we can Supernet its two networks.