2.2.5.2 Classless Inter Domain Routing (CIDR)

The explosive growth of the Internet has forced a review of address assignment policies. The traditional uses of general purpose (Class A, B, and C) networks have been modified to achieve better use of IP's 32-bit address space. Classless Inter Domain Routing (CIDR) [INTERNET:15] is a method currently being deployed in the Internet backbones to achieve this added efficiency. CIDR depends on deploying and routing to arbitrarily sized networks. In this model, hosts and routers make no assumptions about the use of addressing in the internet. The Class D (IP Multicast) and Class E (Experimental) address spaces are preserved, although this is primarily an assignment policy.

By definition, CIDR comprises three elements:

• topologically significant address assignment,
• routing protocols that are capable of aggregating network layer reachability information, and
• consistent forwarding algorithm ("longest match").

The use of networks and subnets is now historical, although the language used to describe them remains in current use. They have been replaced by the more tractable concept of a network prefix. A network prefix is, by definition, a contiguous set of bits at the more significant end of the address that defines a set of systems; host numbers select among those systems. There is no requirement that all the internet use network prefixes uniformly. To collapse routing information, it is useful to divide the internet into addressing domains. Within such a domain, detailed information is available about constituent networks; outside it, only the common network prefix is advertised.

The classical IP addressing architecture used addresses and subnet masks to discriminate the host number from the network prefix. With network prefixes, it is sufficient to indicate the number of bits in the prefix. Both representations are in common use. Architecturally correct subnet masks are capable of being represented using the prefix length description. They comprise that subset of all possible bits patterns that have

• a contiguous string of ones at the more significant end,
• a contiguous string of zeros at the less significant end, and
• no intervening bits.

Routers SHOULD always treat a route as a network prefix, and SHOULD reject configuration and routing information inconsistent with that model.

      IP-address ::= { <Network-prefix>, <Host-number> }

An effect of the use of CIDR is that the set of destinations associated with address prefixes in the routing table may exhibit subset relationship. A route describing a smaller set of destinations (a longer prefix) is said to be more specific than a route describing a larger set of destinations (a shorter prefix); similarly, a route describing a larger set of destinations (a shorter prefix) is said to be less specific than a route describing a smaller set of destinations (a longer prefix). Routers must use the most specific matching route (the longest matching network prefix) when forwarding traffic.