5.1   Administration of IP addresses within a domain

      If individual subnetworks take their IP addresses from a myriad of
      unrelated IP address spaces, there will be effectively no data
      abstraction beyond what is built into existing intra-domain
      routing protocols.  For example, assume that within a routing
      domain uses three independent prefixes assigned from three
      different IP address spaces associated with three different
      attached providers.

      This has a negative effect on inter-domain routing, particularly
      on those other domains which need to maintain routes to this
      domain.  There is no common prefix that can be used to represent
      these IP addresses and therefore no summarization can take place
      at the routing domain boundary. When addresses are advertised by
      this routing domain to other routing domains, an enumerated list
      of the three individual prefixes must be used.

      This situation is roughly analogous to the present dissemination
      of routing information in the Internet, where each domain may have
      non-contiguous network numbers assigned to it.  The result of
      allowing subnetworks within a routing domain to take their IP
      addresses from unrelated IP address spaces is flat routing at the
      A/B/C class network level.  The number of IP prefixes that leaf
      routing domains would advertise is on the order of the number of
      attached network numbers; the number of prefixes a provider's
      routing domain would advertise is approximately the number of
      network numbers attached to the client leaf routing domains; and
      for a backbone this would be summed across all attached providers.
      This situation is just barely acceptable in the current Internet,
      and as the Internet grows this will quickly become intractable. A
      greater degree of hierarchical information reduction is necessary
      to allow continued growth in the Internet.

5.2   Administration at the Leaf Routing Domain

      As mentioned previously, the greatest degree of data abstraction
      comes at the lowest levels of the hierarchy. Providing each leaf
      routing domain (that is, site) with a prefix from its provider's
      prefix results in the biggest single increase in abstraction. From
      outside the leaf routing domain, the set of all addresses



Rekhter & Li                                                    [Page 8]

RFC 1518          CIDR Address Allocation Architecture    September 1993


      reachable in the domain can then be represented by a single
      prefix.  Further, all destinations reachable within the provider's
      prefix can be represented by a single prefix.

      For example, consider a single campus which is a leaf routing
      domain which would currently require 4 different IP networks.
      Under the new allocation scheme, they might instead be given a
      single prefix which provides the same number of destination
      addresses.  Further, since the prefix is a subset of the
      provider's prefix, they impose no additional burden on the higher
      levels of the routing hierarchy.

      There is a close relationship between subnetworks and routing
      domains implicit in the fact that they operate a common routing
      protocol and are under the control of a single administration. The
      routing domain administration subdivides the domain into
      subnetworks.  The routing domain represents the only path between
      a subnetwork and the rest of the internetwork. It is reasonable
      that this relationship also extend to include a common IP
      addressing space. Thus, the subnetworks within the leaf routing
      domain should take their IP addresses from the prefix assigned to
      the leaf routing domain.