RFC 3177 (rfc3177) - Page 3 of 10

IAB/IESG Recommendations on IPv6 Address Allocations to Sites

RFC 3177       IAB/IESG Recommendations on IPv6 Addresses September 2001


   In [RFC 2374] and [RFC 2450], the IETF's IPNG working group has
   recommended that the address block given to a single edge network
   which may be recursively subnetted be a 48-bit prefix.  This gives
   each such network 2^16 subnet numbers to use in routing, and a very
   large number of unique host numbers within each network.  This is
   deemed to be large enough for most enterprises, and to leave plenty
   of room for delegation of address blocks to aggregating entities.

   It is not obvious, however, that all edge networks are likely to be
   recursively subnetted; a single PC in a home or a telephone in a
   mobile cellular network, for example, may or may not interface to a
   subnetted local network.  When a network number is delegated to a
   place that will not require subnetting, therefore, it might be
   acceptable for an ISP to give a single 64-bit prefix - perhaps shared
   among the dial-in connections to the same ISP router.  However this
   decision may be taken in the knowledge that there is objectively no
   shortage of /48s, and the expectation that personal, home networks
   will become the norm.  Indeed, it is widely expected that all IPv6
   subscribers, whether domestic (homes), mobile (vehicles or
   individuals), or enterprises of any size, will eventually possess
   multiple always-on hosts, at least one subnet with the potential for
   additional subnetting, and therefore some internal routing
   capability.  In other words the subscriber allocation unit is not
   always a host; it is always potentially a site.  The question this
   memo is addressing is how much address space should be delegated to
   such sites.

3. Address Delegation Recommendations

   The IESG and the IAB recommend the allocations for the boundary
   between the public and the private topology to follow those general
   rules:

      -  /48 in the general case, except for very large subscribers.
      -  /64 when it is known that one and only one subnet is needed by
         design.
      -  /128 when it is absolutely known that one and only one device
         is connecting.

   In particular, we recommend:

      -  Home network subscribers, connecting through on-demand or
         always-on connections should receive a /48.
      -  Small and large enterprises should receive a /48.
      -  Very large subscribers could receive a /47 or slightly shorter
         prefix, or multiple /48's.





IAB & IESG                   Informational