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Classless Addressing Connected: An Internet Encyclopedia
Classless Addressing

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Up: Programmed Instruction Course
Up: Subnetting and CIDR
Prev: Subnetting
Next: Routing Protocols Evolved

Classless Addressing By 1990, the Internet was facing serious growth pains. The two most severe problems were the explosion of routing table size and the looming exhaustion of Class B networks. The wild popularity of the net had triggered a flood of new classful networks, and every one had to be included in the routing tables. The routers were running out of memory, and spending far too much time doing address lookups. Futhermore, it had become apparent that the pace of requests for new Class B networks would soon exhaust the available supply.

The Internet Engineering Task Force (IETF), recognizing the urgency of these twin problems, assigned the ROAD group to develop a solution. That solution became known as classless routing, supernetting, or CIDR, and is the addressing scheme currently used in the Internet.

CIDR was based on the already successful practice of subnetting. By supernetting, or allowing the subnet boundary to move to the left, into the network portion, both problems could be solved. Groups of neighboring classful networks could be combined into single routing table entries, thus reducing the size of the tables through summarization. Groups of Class C networks could be assigned in batches of 2, 4, 8, or 16 to fill the needs of organizations that would otherwise have requested the increasingly scarse Class Bs. CIDR also eliminated most of subnetting's restrictions.

CIDR is the addressing scheme you've learned in this course.

CIDR, by generalizing the practice of subnetting, closed the lid on the coffin of classful addressing, which had simply proved too inflexible to manage the global Internet. As we'll see, vestiges of the old addressing scheme still haunt network engineers, but the prudent designer, by installing modern routing protocols and following the practices described earlier, will reap all the benefits of CIDR's prefix-based addressing.

Next: Routing Protocols Evolved

Connected: An Internet Encyclopedia
Classless Addressing


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