CIDR

Classless Inter-Domain Routing (CIDR)

Part of the IP address (variable length) represents a network number, the rest of the address is used to number nodes. The system is flexible and uses /x notation to define the length of the network address. For example;

172.16.0.0/12 includes all the networks from 172.16.0.0 to 172.31.0.0 and /12 would be the same as a subnet mask of 255.248.0.0

Suppose we wanted four class C addresses to be aggregated as a single network. One class C address is a /24, two would be a /23 and four would be a /22.

The following table shows a simplistic view of all the possible subnets, their sizes and subnet masks.

This is easy to build in Excel, give it a go!