Classless Inter-Domain Routing (CIDR) is a methodology of allocating IP addresses and routing Internet Protocol packets. It was introduced in 1993 to replace the prior addressing architecture of classful network design in the Internet with the goal 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 part is the network address which identifies a whole network or subnet and the least significant portion is the host identifier, which specifies a particular host interface 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 host identifier has a fixed size of 64 bits by convention, and smaller subnets are never allocated to end users.
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