Classless Inter-Domain Routing (CIDR) is an IP addressing system that replaces an older system based on classes. One IP address can be used to designate several unique IP addresses using CIDR.
A CIDR IP address looks like a normal IP address but it ends with a forward slash followed by a number, call the IP network prefix - e.g. 10.10.10.10/24. CIDR makes more IP addresses available within an organisation.