This type of address is termed an "IPv4-mapped IPv6 address" and has
   the format:




 
RFC 1884              IPv6 Addressing Architecture         December 1995



    |                80 bits               | 16 |      32 bits        |
    +--------------------------------------+--------------------------+
    |0000..............................0000|FFFF|    IPv4 address     |
    +--------------------------------------+----+---------------------+



   2.4.5 NSAP Addresses

   This mapping of NSAP address into IPv6 addresses is as follows:


    |   7   |                   121 bits                              |
    +-------+---------------------------------------------------------+
    |0000001|                 to be defined                           |
    +-------+---------------------------------------------------------+

   The draft definition, motivation, and usage are under study [NSAP].


   2.4.6 IPX Addresses

   This mapping of IPX address into IPv6 addresses is as follows:


    |   7   |                   121 bits                              |
    +-------+---------------------------------------------------------+
    |0000010|                 to be defined                           |
    +-------+---------------------------------------------------------+

   The draft definition, motivation, and usage are under study.


   2.4.7 Provider-Based Global Unicast Addresses

   The global provider-based unicast address is assigned as described in
   [ALLOC].  This initial assignment plan for these unicast addresses is
   similar to assignment of IPv4 addresses under the CIDR scheme [CIDR].
   The IPv6 global provider-based unicast address format is as follows:


    | 3 |  n bits   |  m bits   |   o bits    |   125-n-m-o bits   |
    +---+-----------+-----------+-------------+--------------------+
    |010|registry ID|provider ID|subscriber ID|  intra-subscriber  |
    +---+-----------+-----------+-------------+--------------------+






 
RFC 1884              IPv6 Addressing Architecture         December 1995


   The high-order part of the address is assigned to registries, who
   then assign portions of the address space to providers, who then
   assign portions of the address space to subscribers, etc.

   The registry ID identifies the registry which assigns the provider
   portion of the address.  The term "registry prefix" refers to the
   high-order part of the address up to and including the registry ID.

   The provider ID identifies a specific provider which assigns the
   subscriber portion of the address.  The term "provider prefix" refers
   to the high-order part of the address up to and including the
   provider ID.

   The subscriber ID distinguishes among multiple subscribers attached
   to the provider identified by the provider ID.  The term "subscriber
   prefix" refers to the high-order part of the address up to and
   including the subscriber ID.

   The intra-subscriber portion of the address is defined by an
   individual subscriber and is organized according to the subscribers
   local internet topology.  It is likely that many subscribers will
   choose to divide the intra-subscriber portion of the address into a
   subnet ID and an interface ID.  In this case the subnet ID identifies
   a specific physical link and the interface ID identifies a single
   interface on that subnet.


   2.4.8 Local-use IPv6 Unicast Addresses

   There are two types of local-use unicast addresses defined.  These
   are Link-Local and Site-Local.  The Link-Local is for use on a single
   link and the Site-Local is for use in a single site.  Link-Local
   addresses have the following format: