protocols.  The known boundaries will differ from router to router,
   depending on what positions the router holds in the routing
   hierarchy.


   2.4.1 Unicast Address Examples

   An example of a Unicast address format which will likely be common on
   LANs and other environments where IEEE 802 MAC addresses are
   available is:


    |              n bits            | 80-n bits |     48 bits        |
    +--------------------------------+-----------+--------------------+
    |        subscriber prefix       | subnet ID |   interface ID     |
    +--------------------------------+-----------+--------------------+

   Where the 48-bit Interface ID is an IEEE-802 MAC address.  The use of
   IEEE 802 MAC addresses as a interface ID is expected to be very
   common in environments where nodes have an IEEE 802 MAC address.  In
   other environments, where IEEE 802 MAC addresses are not available,
   other types of link layer addresses can be used, such as E.164
   addresses, for the interface ID.

   The inclusion of a unique global interface identifier, such as an
   IEEE MAC address, makes possible a very simple form of auto-
   configuration of addresses.  A node may discover a subnet ID by
   listening to Router Advertisement messages sent by a router on its
   attached link(s), and then fabricating an IPv6 address for itself by
   using its IEEE MAC address as the interface ID on that subnet.

   Another unicast address format example is where a site or
   organization requires additional layers of internal hierarchy.  In
   this example the subnet ID is divided into an area ID and a subnet
   ID.  Its format is:

    |         s bits       | n bits  |   m bits     | 128-s-n-m bits  |
    +----------------------+---------+--------------+-----------------+
    |   subscriber prefix  | area ID |  subnet ID   |  interface ID   |
    +----------------------+---------+--------------+-----------------+

   This technique can be continued to allow a site or organization to
   add additional layers of internal hierarchy.  It may be desirable to
   use an interface ID smaller than a 48-bit IEEE 802 MAC address to
   allow more space for the additional layers of internal hierarchy.
   These could be interface IDs which are administratively created by




 
RFC 1884              IPv6 Addressing Architecture         December 1995


   the site or organization.


   2.4.2 The Unspecified Address

   The address 0:0:0:0:0:0:0:0 is called the unspecified address.  It
   must never be assigned to any node.  It indicates the absence of an
   address.  One example of its use is in the Source Address field of
   any IPv6 datagrams sent by an initializing host before it has learned
   its own address.

   The unspecified address must not be used as the destination address
   of IPv6 datagrams or in IPv6 Routing Headers.


   2.4.3 The Loopback Address

   The unicast address 0:0:0:0:0:0:0:1 is called the loopback address.
   It may be used by a node to send an IPv6 datagram to itself.  It may
   never be assigned to any interface.

   The loopback address must not be used as the source address in IPv6
   datagrams that are sent outside of a single node.  An IPv6 datagram
   with a destination address of loopback must never be sent outside of
   a single node.


   2.4.4 IPv6 Addresses with Embedded IPv4 Addresses

   The IPv6 transition mechanisms 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.