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What is CIDR Notation

Classless Inter Domain Routing (CIDR) is a method for assigning IP addresses without using the standard IP address classes like Class A, Class B or Class C.

In CIDR notation, an IP address is represented as A.B.C.D /n, where "/n" is called the IP prefix or network prefix. The IP prefix identifies the number of significant bits used to identify a network. For example, 192.9.205.22 /18 means, the first 18 bits are used to represent the network and the remaining 14 bits are used to identify hosts. Common prefixes are 8, 16, 24, and 32.

Refer to the following page for the CIDR to Netmask Translation Table.

CIDR
Netmask (Dot Notation) Number of Hosts
/1 128.0.0.0  
/2 192.0.0.0  
/3 224.0.0.0  
/4 240.0.0.0  
/5 248.0.0.0  
/6 252.0.0.0  
/7 254.0.0.0  
/8 255.0.0.0  
/9 255.128.0.0  
/10 255.192.0.0  
/11 255.224.0.0  
/12 255.240.0.0  
/13 255.248.0.0  
/14 255.252.0.0  
/15 255.254.0.0  
/16 255.255.0.0  
/17 255.255.128  
/18 255.255.192.0  
/19 255.255.224.0  
/20 255.255.240.0  
/21 255.255.248.0  
/22 255.255.252.0  
/23 255.255.254.0  
/24 255.255.255.0 256
/25 255.255.255.128 128
/26 255.255.255.192 64
/27 255.255.255.224 32
/28 255.255.255.240 16
/29 255.255.255.248 8
/30 255.255.255.252 4
/31 255.255.255.254 2
/32 255.255.255.255 1


next up previous contents
Next: Rule Configuration Up: Configuring IDDS Previous: General Operation   Contents
08-01