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CIDR notation

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See also: Classless Inter-Domain Routing for a more comprehensive description. The current article explains just the notation.

CIDR notation is a compact form for a block of Internet IP Addresses. It is written by adding a "prefix length" to the standard "dotted decimal" notation for 32-bit IPv4 addresses, and has been adapted to IPv6.

IPv4

 192.168.33.32/28
       IP      Block

The /28 in this "CIDR block" indicates a block of 16 addresses. The number 28 is the number of bits in the "network prefix", the leading bits that are common to all addresses in the block.

CIDR blocks are limited to sizes that are a power of two. /32=1, /31=2, 4, 8, 16, ... /24=256, ... /0=2^32 (the entire Internet). The boundaries of the block must also align with all other blocks the same size or larger, e.g. 192.168.33.43/28 is the same block as above.

 192.168.33.32/32      1 address
 192.168.33.32/31      2 addresses
 192.168.33.32/28     16 addresses 
 192.168.33.32/24    256 addresses. C block
 192.168.33.32/16 65,536 addresses. B block

CIDR blocks are commonly used in allocation of network addresses, and the notation is understood by most students of computer networks, but it is not the most flexible or readable notation. The block above could have been shown with a notation like 192.168.33.32-47. The addresses included in the block would then be more clear, and there would be more flexibility in designating odd-sized blocks. The network scanning program NMAP can designate address ranges such as 192.168.0-255.1-254, and have a scan skip all addresses where the last octet is 0 or 255. A range like 0-255.0-255.13.37 could be used to sample scan the entire Internet.

IPv6