2.  CIDR address allocation

   There are two basic components of this addressing and routing plan:
   one, to distribute the allocation of Internet address space and two,
   to provide a mechanism for the aggregation of routing information.

   2.1  Aggregation and its limitations

   One major goal of this addressing plan is to allocate Internet
   address space in such a manner as to allow aggregation of routing



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RFC 1519                 CIDR Address Strategy            September 1993


   information along topological lines.  For simple, single-homed
   clients, the allocation of their address space out of a transit
   routing domain's space will accomplish this automatically - rather
   than advertise a separate route for each such client, the transit
   domain may advertise a single aggregate route which describes all of
   the destinations connected to it. Unfortunately, not all sites are
   singly-connected to the network, so some loss of ability to aggregate
   is realized for the non-trivial cases.

   There are two situations that cause a loss of aggregation efficiency.

      o    Organizations which are multi-homed. Because multi-homed
           organizations must be advertised into the system by each of
           their service providers, it is often not feasible to
           aggregate their routing information into the address space
           any one of those providers. Note that they still may receive
           their address allocation out of a transit domain's address
           space (which has other advantages), but their routing
           information must still be explicitly advertised by most of
           their service providers (the exception being that if the
           site's allocation comes out of its least-preferable service
           provider, then that service provider need not advertise the
           explicit route - longest-match will insure that its
           aggregated route is used to get to the site on a backup
           basis).  For this reason, the routing cost for these
           organizations will typically be about the same as it is
           today.

      o    Organizations which change service provider but do not
           renumber. This has the effect of "punching a hole" in the
           aggregation of the original service provider's advertisement.
           This plan will handle the situation by requiring the newer
           service provider to advertise a specific advertisement for
           the new client, which is preferred by virtue of being the
           longest match.  To maintain efficiency of aggregation, it is
           recommended that organizations which do change service
           providers plan to eventually migrate their address
           assignments from the old provider's space to that of the new
           provider. To this end, it is recommended that mechanisms to
           facilitate such migration, including improved protocols and
           procedures for dynamic host address assignment, be developed.

   Note that some aggregation efficiency gain can still be had for
   multi-homed sites (and, in general, for any site composed of
   multiple, logical IP network numbers) - by allocating a contiguous
   power-of-two block of network numbers to the client (as opposed to
   multiple, independent network numbers) the client's routing
   information may be aggregated into a single (net, mask) pair. Also,



Fuller, Li, Yu & Varadhan                                       [Page 4]

RFC 1519                 CIDR Address Strategy            September 1993


   since the routing cost associated with assigning a multi-homed site
   out of a service provider's address space is no greater than the
   current method of a random allocation by a central authority, it
   makes sense to allocate all address space out of blocks assigned to
   service providers.

   It is also worthwhile to mention that since aggregation may occur at
   multiple levels in the system, it may still be possible to aggregate
   these anomalous routes at higher levels of whatever hierarchy may be
   present. For example, if a site is multi-homed to two NSFNET regional
   networks both of whom obtain their address space from the NSFNET,
   then aggregation by the NSFNET of routes from the regionals will
   include all routes to the multi-homed site.

   Finally, it should also be noted that deployment of the new
   addressing plan described in this document may (and should) begin
   almost immediately but effective use of the plan to aggregate routing
   information will require changes to some Inter-Domain routing
   protocols. Likewise, deploying classless Inter-Domain protocols
   without deployment of the new address plan will not allow useful
   aggregation to occur (in other words, the addressing plan and routing
   protocol changes are both required for supernetting, and its
   resulting reduction in table growth, to be effective.)  Note,
   however, that during the period of time between deployment of the
   addressing plan and deployment of the new protocols, the size of
   routing tables may temporarily grow very rapidly. This must be
   considered when planning the deployment of the two plans.

   Note: in the discussion and examples which follow, the network and
   mask notation is used to represent routing destinations. This is used
   for illustration only and does not require that routing protocols use
   this representation in their updates.