# CIDR notation

Main Article
Discussion
Related Articles  [?]
Bibliography  [?]
Citable Version  [?]

This editable Main Article is under development and subject to a disclaimer.
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