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What Is CIDR?


CIDR is short for Classless Inter-Domain Routing.

CIDR is an 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 network prefix. For example:

172.200.0.0/16

The IP network prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP network prefix of /12, for example, can be used to address 1,048,576 former Class C addresses.

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

CIDR is also called supernetting.

CIDR Block Prefix # Equivalent Class C # of Host
/27 1/8th of a Class C 32 hosts
/26 1/4th of a Class C 64 hosts
/25 1/2 of a Class C 128 hosts
/24 1 Class C 256 hosts
/23 2 Class C 512 hosts
/22 4 Class C 1,024 hosts
/21 8 Class C 2,048 hosts
/20
16 Class C
4,096 hosts
/19
32 Class C
8,192 hosts
/18
64 Class C
16,384 hosts
/17
128 Class C
32,768 hosts
/16
256 Class C (= 1 Class B)
65,536 hosts
/15
512 Class C
131,072 hosts
/14
1,024 Class C
262,144 hosts
/13 2,048 Class C 524,288 hosts



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