Classless Inter-Domain Routing (CIDR) notation is an Internet Protocol (IP) address specification[1] used in the Internet addressing architecture that replaced the classful network organization of the IP address space and is used today for IPv4 as well as IPv6 networking.
The notation specifies an Internet Protocol address and the number of bits of its routing or subnet prefix. The IP address is written according to the standards of IPv4 or IPv6. It is followed by a separator character, the forward slash (/) and a decimal number, the size of the prefix.
The address may denote a single, distinct interface address or the beginning address of an entire network. In the latter case the CIDR notation specifies the address block allocation of the network. The maximum size of the network is given by the number of addresses that are possible with the remaining least-significant bits below the prefix (host identifier).
For example:
- the address specification 192.168.100.1/24 represents the given IPv4 address and its associated routing prefix (192.168.100.0) or, equivalently, its subnet mask, 255.255.255.0.
- the IPv4 block 192.168.0.0/22 represents the 1024 IPv4 addresses from 192.168.0.0 to 192.168.3.255.
- the IPv6 block 2001:DB8::/48 represents the IPv6 addresses from 2001:DB8:0:0:0:0:0:0 to 2001:DB8:0:FFFF:FFFF:FFFF:FFFF:FFFF.
- ::1/128 represents the IPv6 loopback address.
For IPv4 networks, compare with dot-decimal notation, an alternative representation which uses the network address followed by the network's subnet mask, written as:
- 192.168.0.0/24 could be written 192.168.0.0/255.255.255.0
- 192.168.0.0/22 could be written 192.168.0.0/255.255.252.0
The number of addresses of a subnet defined by the mask or prefix can be calculated as 2address size - mask. For example, a mask of /29 gives: 232-29 = 23 = 8 addresses for an IPv4 network. However, because at least one of these addresses is typically used for a gateway to other subnets, and because certain addresses are reserved as broadcast addresses, the number of addresses available for hosts is usually smaller.
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