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RFC 1817

                       CIDR and Classful Routing


   Classless Inter-Domain Routing (CIDR) ([RFC1518], [RFC1519]) is
   deployed in the Internet as the primary mechanism to improve scaling
   property of the Internet routing system. Essential to CIDR is the
   generalization of the concept of variable length subnet masks (VLSM)
   and the elimination of classes of network numbers (A, B, and C). The
   interior (intra-domain) routing protocols that support CIDR are OSPF,
   RIP II, Integrated IS-IS, and E-IGRP.  The exterior (inter-domain)
   routing protocol that supports CIDR is BGP-4.  Protocols like RIP,
   BGP-3, EGP, and IGRP do not support CIDR.

Implications of CIDR

   Deployment of CIDR has certain implications on the segments of the
   Internet that are still using routing technology that can not support
   CIDR. Existing sites that rely solely on a default route for their
   external connectivity may not require support of VLSM capable routing
   technology for their interior routing and CIDR for their exterior
   routing. All sites lacking support for VLSM and CIDR capable routing
   must rely on a default route, which consequently may result in a
   various degree of suboptimal routing.  Organizations that operate as
   Internet Service Providers (ISPs) are expected to be able to support
   VLSM and CIDR.

Rekhter                      Informational                      [Page 1]

RFC 1817 CIDR and Classful Routing August 1995 It is expected that in the near future the IANA will instruct the Internet Registries to begin allocating IP addresses out of the former Class A address space ( through The allocated blocks are going to be of variable size (based on the actual sites' requirements). Sites that will use these addresses will have to support CIDR-capable routing protocols. All the providers will be required to support CIDR-capable routing protocols as well. Sites that do not use these addresses would be required to continue relying on a default route, which in turn may result in a various degree of suboptimal routing. If a site wants to avoid the suboptimality (introduced by using default route), the site will need to transition to CIDR-capable routing protocols.

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