What is CIDR?

Classless Inter Domain Routing, replaces the subnet class system.
Subnets are defined by the number of binary bits they have in common.
Each octet of the IP address is broken down into two hexadecimal digits;
8 bits represent each pair of hexadecimal digits.
class A 8 bit subnet (first octet).
class B 16 bit subnet (first and second octets).
class C 24 bit subnet (first, second, and third octets).