2.2 Classless Interdomain Routing
2.2.1 Why classless interdomain routing?
A relatively recent addition to the IP address architecture is CIDR (pronounced "Cider"). CIDR was born of the crisis that accompanied the Internet's explosive growth during the early 1990s.

As early as 1992, the IETF became concerned with the Internet's capability to continue to scale upward in response to demand for Internet use. Their specific concerns were as follows:

  • Exhaustion of the remaining, unassigned IPv4 network addresses. The Class B space was in particular danger of depletion.
  • The rapid, and substantial, increase in the size of the Internet's routing tables as a result of its growth.

All the indications were that the Internet's rapid growth would continue, as more commercial organizations came online. In fact, some members of the IETF even predicted a "Date of Doom". This date, March 1994, was the projected date of the depletion of the Class B address space. Absent any other mechanism for addressing, the Internet's scalability would be seriously compromised. More ominously, the Internet's routing mechanisms might collapse under the weight of their ever-growing routing tables before the "Date of Doom".

The Internet was becoming a victim of its own success. The IETF decided that, to avoid the collapse of the Internet, both short- and long-term solutions would be needed. In the long term, the only viable solution was a completely new IP, with greatly expanded address space and address architectures. Ultimately, this solution became known as IPng (Internet Protocol: The Next Generation) or, more formally, as IP Version 6 (IPv6).

The more pressing, short-term needs were to slow down the rate of depletion of the remaining unassigned addresses. The answer was to eliminate the inefficient classes of addresses in favor of a more flexible addressing architecture. The result was CIDR.

In September 1993, the plans for CIDR were released in RFCs 1517, 1518, 1519, and 1520. CIDR had three key features that were invaluable in staving off depletion of the IPv4 address space. These features are the following:

  • The replacement of classful addressing by classless
  • Enhanced route aggregation
  • Supernetting