Select Page

A system called Classless Inter-Domain Routing, or CIDR, was developed as an alternative to traditional subnetting. The idea is that you can add a specification in the IP address itself as to the number of significant bits that make up the routing or networking portion.

For example, we could express the idea that the IP address 192.168.0.15 is associated with the netmask 255.255.255.0 by using the CIDR notation of 192.168.0.15/24. This means that the first 24 bits of the IP address given are considered significant for the network routing.

This allows us some interesting possibilities. We can use these to reference “supernets”. In this case, we mean a more inclusive address range that is not possible with a traditional subnet mask. For instance, in a class C network, like above, we could not combine the addresses from the networks 192.168.0.0 and 192.168.1.0 because the netmask for class C addresses is 255.255.255.0.

However, using CIDR notation, we can combine these blocks by referencing this chunk as 192.168.0.0/23. This specifies that there are 23 bits used for the network portion that we are referring to.

So the first network (192.168.0.0) could be represented like this in binary:

While the second network (192.168.1.0) would be like this:

The CIDR address we specified indicates that the first 23 bits are used for the network block we are referencing. This is equivalent to a netmask of 255.255.254.0, or:

As you can see, with this block the 24th bit can be either 0 or 1 and it will still match, because the network block only cares about the first 23 digits.

Basically, CIDR allows us more control over addressing continuous blocks of IP addresses. This is much more useful than the subnetting we talked about originally.