CIDR

Classless Inter-Domain Routing

CIDR was created to allow for greater flexibility with routed IP networks, to allow for accelerating expansion of the Internet.

A definition of a CIDR block includes a network number(in 4 octets? for IPv4, and a variable length set of up to 8 16-bit hexidecimal fields for IPv6), and a prefix length, in bits, defining the size of the network in question.

For example: 192.168.0.0/24 represents the IPv4 addresses 192.168.0.0 through 192.168.0.255 inclusive, with 192.168.0.255 being the broadcast address for the network. 192.168.0.0/22 represents the IPv4 addresses 192.168.0.0 through 192.168.3.255 inclusive, with 192.168.3.255 being the broadcast address for the network. 2002:C0A8::/48 represents the IPv6 addresses 2002:C0A8:0:0:0:0:0:0 through 2002:C0A8:0:FFFF:FFFF:FFFF:FFFF:FFFF, inclusive.

CIDR network addresses are used in many places, including routers and servers both connected and unconnected to the Internet, though an average desktop user generally does not see them in practice.