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CIDR Short for Classless Inter-Domain Routing, a new IP addressing scheme that replaces the older system based on classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP prefix. For example:

The IP prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP prefix of /12, for example, can be used to address 4,096 former Class C addresses.

CIDR addresses reduce the size of routing tables and make more IP addresses available within organizations.