Classless Inter-Domain Routing
Classless Inter-Domain Routing (CIDR, pronunciation: /’s??.dr/ or /’si.dr/) is a method for allocating IP addresses and routing Internet Protocol packets. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of classful network design in the Internet. Its goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.
IP addresses are described as consisting of two groups of bits in the address: the most significant bits are the network address, which identifies a whole network or subnet, and the least significant set forms the host identifier, which specifies a particular interface of a host on that network. This division is used as the basis of traffic routing between IP networks and for address allocation policies. Classful network design for IPv4 sized the network address as one or more 8-bit groups, resulting in the blocks of Class A, B, or C addresses. Classless Inter-Domain Routing allocates address space to Internet service providers and end users on any address bit boundary, instead of on 8-bit segments. In IPv6, however, the interface identifier
has a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.
CIDR notation is a syntax for specifying IP addresses and their associated routing prefix. It appends a slash character to the address and the decimal number of leading bits of the routing
prefix, e.g., 192.168.2.0/24 for IPv4, and 2001:db8::/32 for IPv6.