RFC 2373              IPv6 Addressing Architecture             July 1998


2.5.4 IPv6 Addresses with Embedded IPv4 Addresses

   The IPv6 transition mechanisms [TRAN] include a technique for hosts
   and routers to dynamically tunnel IPv6 packets over IPv4 routing
   infrastructure.  IPv6 nodes that utilize this technique are assigned
   special IPv6 unicast addresses that carry an IPv4 address in the low-
   order 32-bits.  This type of address is termed an "IPv4-compatible
   IPv6 address" and has the format:

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

   A second type of IPv6 address which holds an embedded IPv4 address is
   also defined.  This address is used to represent the addresses of
   IPv4-only nodes (those that *do not* support IPv6) as IPv6 addresses.
   This type of address is termed an "IPv4-mapped IPv6 address" and has
   the format:

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

2.5.5 NSAP Addresses

   This mapping of NSAP address into IPv6 addresses is defined in
   [NSAP].  This document recommends that network implementors who have
   planned or deployed an OSI NSAP addressing plan, and who wish to
   deploy or transition to IPv6, should redesign a native IPv6
   addressing plan to meet their needs.  However, it also defines a set
   of mechanisms for the support of OSI NSAP addressing in an IPv6
   network.  These mechanisms are the ones that must be used if such
   support is required.  This document also defines a mapping of IPv6
   addresses within the OSI address format, should this be required.

2.5.6 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.





 
RFC 2373              IPv6 Addressing Architecture             July 1998


2.5.7 Aggregatable Global Unicast Addresses

   The global aggregatable global unicast address is defined in [AGGR].
   This address format is designed to support both the current provider
   based aggregation and a new type of aggregation called exchanges.
   The combination will allow efficient routing aggregation for both
   sites which connect directly to providers and who connect to
   exchanges.  Sites will have the choice to connect to either type of
   aggregation point.

   The IPv6 aggregatable global unicast address format is as follows:

   | 3|  13 | 8 |   24   |   16   |          64 bits               |
   +--+-----+---+--------+--------+--------------------------------+
   |FP| TLA |RES|  NLA   |  SLA   |         Interface ID           |
   |  | ID  |   |  ID    |  ID    |                                |
   +--+-----+---+--------+--------+--------------------------------+

   Where

      001          Format Prefix (3 bit) for Aggregatable Global
                   Unicast Addresses
      TLA ID       Top-Level Aggregation Identifier
      RES          Reserved for future use
      NLA ID       Next-Level Aggregation Identifier
      SLA ID       Site-Level Aggregation Identifier
      INTERFACE ID Interface Identifier

   The contents, field sizes, and assignment rules are defined in
   [AGGR].

2.5.8 Local-Use IPv6 Unicast Addresses

   There are two types of local-use unicast addresses defined.  These