dropped.





 
RFC 3513              IPv6 Addressing Architecture            April 2003


2.5.4 Global Unicast Addresses

   The general format for IPv6 global unicast addresses is as follows:

   |         n bits         |   m bits  |       128-n-m bits         |
   +------------------------+-----------+----------------------------+
   | global routing prefix  | subnet ID |       interface ID         |
   +------------------------+-----------+----------------------------+

   where the global routing prefix is a (typically hierarchically-
   structured) value assigned to a site (a cluster of subnets/links),
   the subnet ID is an identifier of a link within the site, and the
   interface ID is as defined in section 2.5.1.

   All global unicast addresses other than those that start with binary
   000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as
   described in section 2.5.1.  Global unicast addresses that start with
   binary 000 have no such constraint on the size or structure of the
   interface ID field.

   Examples of global unicast addresses that start with binary 000 are
   the IPv6 address with embedded IPv4 addresses described in section
   2.5.5 and the IPv6 address containing encoded NSAP addresses
   specified in [NSAP].  An example of global addresses starting with a
   binary value other than 000 (and therefore having a 64-bit interface
   ID field) can be found in [AGGR].

2.5.5 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 use this technique are assigned
   special IPv6 unicast addresses that carry a global 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     |
   +--------------------------------------+----+---------------------+

   Note: The IPv4 address used in the "IPv4-compatible IPv6 address"
   must be a globally-unique IPv4 unicast address.

   A second type of IPv6 address which holds an embedded IPv4 address is
   also defined.  This address type is used to represent the addresses
   of IPv4 nodes as IPv6 addresses.  This type of address is termed an
   "IPv4-mapped IPv6 address" and has the format:




 
RFC 3513              IPv6 Addressing Architecture            April 2003


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

2.5.6 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:

   |   10     |
   |  bits    |         54 bits         |          64 bits           |
   +----------+-------------------------+----------------------------+
   |1111111010|           0             |       interface ID         |
   +----------+-------------------------+----------------------------+

   Link-Local addresses are designed to be used for addressing on a
   single link for purposes such as automatic address configuration,
   neighbor discovery, or when no routers are present.

   Routers must not forward any packets with link-local source or
   destination addresses to other links.

   Site-Local addresses have the following format:

   |   10     |
   |  bits    |         54 bits         |         64 bits            |
   +----------+-------------------------+----------------------------+
   |1111111011|        subnet ID        |       interface ID         |
   +----------+-------------------------+----------------------------+

   Site-local addresses are designed to be used for addressing inside of