Classless Inter-Domain Routing: Difference between revisions

Revision as of 00:08, 7 March 2004

Revision as of 11:49, 24 March 2004

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A variable length subnet mask is a means of allocating IP addressing resources to subnets according to their individual need rather than some general network-wide rule. So therefore the network/host division can occur at any bit boundary in the address. Because the normal class distinctions are ignored, the new system was called '''classless routing'''. This led to the original system being called, by back-formation, '''classful routing'''. Classless routing came into use in the mid [[1990s]] due to the inefficiences of the classful system.

Another purpose of CIDR was the possibility of '''routing prefix aggregation''': for example, sixteen contiguous /24 networks could now be aggregated together, and advertised to the outside world as a single /20 route. Two contiguous /20s could then be aggregated to a /19, and so forth. This allowed a significant reduction in the number of routes that had to be advertised over the Internet, preventing 'routing table explosion' from overwhelmoverwhelming routers and stopping the Internet from expanding further.

Nowadays most [[ISP]]s on the public Internet will not route anything smaller than a /19 prefix, effectively preventing small networks from obtaining full public Internet routing without going through a routing aggregator such as an ISP.