IPv6ユニキャストアドレス割り当て体系（IPv6アドレスとルーティング）

   For the purposes of this paper, an IPv6 address prefix is defined as
   an IPv6 address and some indication of the leftmost contiguous
   significant bits within this address portion.  Throughout this paper
   IPv6 address prefixes will be represented as X/Y, where X is a prefix
   of an IPv6 address in length greater than or equal to that specified
   by Y and Y is the (decimal) number of the leftmost contiguous
   significant bits within this address.  In the notation, X, the prefix
   of an IPv6 address [2] will have trailing insignificant digits
   removed.  Thus, an IPv6 prefix might appear to be 43DC:0A21:76/40.

   When determining an administrative policy for IPv6 address
   assignment, it is important to understand the technical consequences.
   The objective behind the use of hierarchical routing is to achieve
   some level of routing data abstraction, or summarization, to reduce
   the cpu, memory, and transmission bandwidth consumed in support of
   routing.

   While the notion of routing data abstraction may be applied to
   various types of routing information, this paper focuses on one
   particular type, namely reachability information. Reachability
   information describes the set of reachable destinations.  Abstraction
   of reachability information dictates that IPv6 addresses be assigned
   according to topological routing structures. However in practice
   administrative assignment falls along organizational or political
   boundaries. These may not be congruent to topological boundaries and
   therefore the requirements of the two may collide. It is necessary to
   find a balance between these two needs.

   Reachability information abstraction occurs at the boundary between
   hierarchically arranged topological routing structures. An element
   lower in the hierarchy reports summary reachability information to
   its parent(s).

   At routing domain boundaries, IPv6 address information is exchanged
   (statically or dynamically) with other routing domains. If IPv6
   addresses within a routing domain are all drawn from non-contiguous
   IPv6 address spaces (allowing no abstraction), then the address
   information exchanged at the boundary consists of an enumerated list
   of all the IPv6 addresses.

   Alternatively, should the routing domain draw IPv6 addresses for all
   the hosts within the domain from a single IPv6 address prefix,
   boundary routing information can be summarized into the single IPv6
   address prefix.  This permits substantial data reduction and allows
   better scaling (as compared to the uncoordinated addressing discussed
   in the previous paragraph).

   If routing domains are interconnected in a more-or-less random (i.e.,
   non-hierarchical) scheme, it is quite likely that no further
   abstraction of routing data can occur. Since routing domains would
   have no defined hierarchical relationship, administrators would not
   be able to assign IPv6 addresses within the domains out of some
   common prefix for the purpose of data abstraction. The result would
   be flat inter-domain routing; all routing domains would need explicit
   knowledge of all other routing domains that they route to.  This can
   work well in small and medium sized internets.  However, this does
   not scale to very large internets.  For example, we expect IPv6 to
   grow to hundreds of thousands of routing domains in North America
   alone.  This requires a greater degree of the reachability
   information abstraction beyond that which can be achieved at the
   `routing domain' level.

   In the Internet, it should be possible to significantly constrain the
   volume and the complexity of routing information by taking advantage
   of the existing hierarchical interconnectivity. This is discussed
   further in Section 5. Thus, there is the opportunity for a group of
   routing domains each to be assigned an address prefix from a shorter
   prefix assigned to another routing domain whose function is to
   interconnect the group of routing domains. Each member of the group
   of routing domains now has its (somewhat longer) prefix, from which
   it assigns its addresses.

   The most straightforward case of this occurs when there is a set of
   routing domains which are all attached to a single service provider
   domain (e.g., regional network), and which use that provider for all
   external (inter-domain) traffic.  A short prefix may be given to the
   provider, which then gives slightly longer prefixes (based on the
   provider's prefix) to each of the routing domains that it
   interconnects. This allows the provider, when informing other routing
   domains of the addresses that it can reach, to abstract the
   reachability information for a large number of routing domains into a
   single prefix. This approach therefore can allow a great deal of
   reduction of routing information, and thereby can greatly improve the
   scalability of inter-domain routing.

   Clearly, this approach is recursive and can be carried through
   several iterations. Routing domains at any `level' in the hierarchy
   may use their prefix as the basis for subsequent suballocations,
   assuming that the IPv6 addresses remain within the overall length and
   structure constraints.

   At this point, we observe that the number of nodes at each lower
   level of a hierarchy tends to grow exponentially. Thus the greatest
   gains in the reachability information abstraction (for the benefit of
   all higher levels of the hierarchy) occur when the reachability
   information aggregation occurs near the leaves of the hierarchy; the
   gains drop significantly at each higher level. Therefore, the law of
   diminishing returns suggests that at some point data abstraction
   ceases to produce significant benefits.  Determination of the point
   at which data abstraction ceases to be of benefit requires a careful
   consideration of the number of routing domains that are expected to
   occur at each level of the hierarchy (over a given period of time),
   compared to the number of routing domains and address prefixes that
   can conveniently and efficiently be handled via dynamic inter-domain
   routing protocols.

   If the Internet plans to support a decentralized address
   administration, then there is a balance that must be sought between
   the requirements on IPv6 addresses for efficient routing and the need
   for decentralized address administration.  A coherent addressing plan
   at any level within the Internet must take the alternatives into
   careful consideration.

   As an example of administrative decentralization, suppose the IPv6
   address prefix 43/8 identifies part of the IPv6 address space
   allocated for North America. All addresses within this prefix may be
   allocated along topological boundaries in support of increased data
   abstraction.  Within this prefix, addresses may be allocated on a
   per-provider bases, based on geography or some other topologically
   significant criteria.  For the purposes of this example, suppose that
   this prefix is allocated on a per-provider basis.  Subscribers within
   North America use parts of the IPv6 address space that is underneath
   the IPv6 address space of their service providers.  Within a routing
   domain addresses for subnetworks and hosts are allocated from the
   unique IPv6 prefix assigned to the domain according to the addressing
   plan for that domain.