Classless Inter-Domain Routing (CIDR)

Classless Inter-Domain Routing (CIDR) is simplified method of

representing a subnet mask.

CIDR identifies the number of binary bits set to a 1 (or on) in a subnet mask, preceded by a slash.

Look at the following subnet mask:

255.255.255.240

Looking at the above subnet mask in binary:

11111111.11111111.11111111.11110000

The first 28 bits of the above subnet mask are set to 1. To represent this in

CIDR notation: /28

Now look at the next example:

192.168.1.1 255.255.255.0

The above address/subnet mask can be represented as follows using CIDR:

192.168.1.1 /24