Classless Inter-Domain Routing (CIDR) / Supernetting

By 1992, the exponential growth of the Internet was beginning to raise serious concerns among members of the IETF about the ability of the Internet's routing system to scale and support future growth. These problems were related to:

• The near-term exhaustion of the Class B network address space
• The rapid growth in the size of the global Internet's routing tables
• The eventual exhaustion of the 32-bit IPv4 address space

Projected Internet growth figures made it clear that the first two problems were likely to become critical by 1994 or 1995. The response to these immediate challenges was the development of the concept of Supernetting or Classless Inter-Domain Routing (CIDR).

The third problem, which is of a more long-term nature, is currently being explored by the IP Next Generation (IPng or IPv6) working group of the IETF.

CIDR was officially documented in September 1993 in RFC 1517, 1518, 1519, and 1520.
CIDR supports two important features that benefit the global Internet routing system:

• CIDR eliminates the traditional concept of Class A, Class B, and Class C network addresses. This enables the efficient allocation of the IPv4 address space which will allow the continued growth of the Internet until IPv6 is deployed.

• CIDR supports route aggregation where a single routing table entry can represent the address space of perhaps thousands of traditional classful routes. This allows a single routing table entry to specify how to route traffic to many individual network addresses. Route aggregation helps control the amount of routing information in the Internet's backbone routers, reduces route flapping (rapid changes in route availability), and eases the local administrative burden of updating external routing information.

Without the rapid deployment of CIDR in 1994 and 1995, the Internet routing tables would have in excess of 70,000 routes (instead of the current 30,000+) and the Internet would probably not be functioning today!

For example read the post Supernetting / Classless Inter-Domain Routing (CIDR) Example.

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