2.2  Distributed allocation of address space

   The basic idea of the plan is to allocate one or more blocks of Class
   C network numbers to each network service provider. Organizations
   using the network service provider for Internet connectivity are
   allocated bitmask-oriented subsets of the provider's address space as
   required.

   It is also worthwhile to mention that once inter-domain protocols
   which support classless network destinations are widely deployed, the
   rules described by this plan generalize to permit arbitrary
   super/subnetting of the remaining class A and class B address space
   (the assumption being that classless inter-domain protocols will
   either allow for non-contiguous subnets to exist in the system or
   that all components of a sub-allocated class A/B will be contained



Fuller, Li, Yu & Varadhan                                       [Page 5]

RFC 1519                 CIDR Address Strategy            September 1993


   within a single routing domain). This will allow this plan to
   continue to be used in the event that the class C space is exhausted
   before implementation of a long-term solution is deployed.  This
   alternative is discussed further below in section 6.

   Hierarchical sub-allocation of addresses in this manner implies that
   clients with addresses allocated out of a given service provider are,
   for routing purposes, part of that service provider and will be
   routed via its infrastructure. This implies that routing information
   about multi-homed organizations, i.e., organizations connected to
   more than one network service provider, will still need to be known
   by higher levels in the hierarchy.

   The advantages of hierarchical assignment in this fashion are

      a)  It is expected to be easier for a relatively small number of
          service providers to obtain addresses from the central
          authority, rather than a much larger, and monotonically
          increasing, number of individual clients.  This is not to be
          considered as a loss of part of the service providers' address
          space.

      b)  Given the current growth of the Internet, a scalable and
          delegatable method of future allocation of network numbers has
          to be achieved.

   For these reasons, and in the interest of providing a consistent
   procedure for obtaining Internet addresses, it is recommended that
   most, if not all, network numbers be distributed through service
   providers.  These issues are discussed in much greater length in [2].

3.  Cost-benefit analysis

   This new method of assigning address through service providers can be
   put into effect immediately and will, from the start, have the
   benefit of distributing the currently centralized process of
   assigning new addresses. Unfortunately, before the benefit of
   reducing the size of globally-known routing destinations can be
   achieved, it will be necessary to deploy an Inter-Domain routing
   protocol capable of handling arbitrary network and mask pairs. Only
   then will it be possible to aggregate individual class C networks
   into larger blocks represented by single routing table entries.

   This means that upon introduction, the new addressing allocation plan
   will not in and of itself help solve the routing table size problem.
   Once the new Inter-Domain routing protocol is deployed, however, an
   immediate drop in the number of destinations which clients of the new
   protocol must carry will occur.  A detailed analysis of the magnitude



Fuller, Li, Yu & Varadhan                                       [Page 6]

RFC 1519                 CIDR Address Strategy            September 1993


   of this expected drop and the permanent reduction in rate of growth
   is given in the next section.

   In should also be noted that the present method of flat address
   allocations imposes a large bureaucratic cost on the central address
   allocation authority. For scaling reasons unrelated to address space
   exhaustion or routing table overflow, this should be changed. Using
   the mechanism proposed in this paper will have the fortunate side
   effect of distributing the address allocation procedure, greatly
   reducing the load on the central authority.

   3.1  Present Allocation Figures

   An informal analysis of "network-contacts.txt" (available from the
   DDN NIC) indicates that as of 2/25/92, 46 of 126 class A network
   numbers have been allocated (leaving 81) and 5467 of 16382 class B
   numbers have been allocated, leaving 10915. Assuming that recent
   trends continue, the number of allocated class B's will continue to
   double approximately once a year.  At this rate of growth, all class
   B's will be exhausted within about 15 months.  As of 1/13/93, 52
   class A network numbers have been allocated and 7133 class B's have
   been allocated.  We suggest that the change in the class B allocation
   rate is due to the initial deployment of this address allocation
   plan.



























Fuller, Li, Yu & Varadhan                                       [Page 7]