Provider-Based Unicast Address     010            1/8

        Unassigned                         011            1/8

        Reserved for Geographic-
        Based Unicast Addresses            100            1/8

        Unassigned                         101            1/8
        Unassigned                         110            1/8
        Unassigned                         1110           1/16
        Unassigned                         1111 0         1/32
        Unassigned                         1111 10        1/64
        Unassigned                         1111 110       1/128

        Unassigned                         1111 1110 0    1/512

        Link Local Use Addresses           1111 1110 10   1/1024
        Site Local Use Addresses           1111 1110 11   1/1024

        Multicast Addresses                1111 1111      1/256

        Note: The "unspecified address" (see section 2.4.2), the
        loopback address (see section 2.4.3), and the IPv6 Addresses
        with Embedded IPv4 Addresses (see section 2.4.4), are assigned
        out of the 0000 0000 format prefix space.


   This allocation supports the direct allocation of provider addresses,
   local use addresses, and multicast addresses.  Space is reserved for
   NSAP addresses, IPX addresses, and geographic addresses.  The
   remainder of the address space is unassigned for future use.  This
   can be used for expansion of existing use (e.g., additional provider
   addresses, etc.) or new uses (e.g., separate locators and
   identifiers).  Fifteen percent of the address space is initially




 
RFC 1884              IPv6 Addressing Architecture         December 1995


   allocated.  The remaining 85% is reserved for future use.

   Unicast addresses are distinguished from multicast addresses by the
   value of the high-order octet of the addresses: a value of FF
   (11111111) identifies an address as a multicast address; any other
   value identifies an address as a unicast address.  Anycast addresses
   are taken from the unicast address space, and are not syntactically
   distinguishable from unicast addresses.


   2.4 Unicast Addresses

   The IPv6 unicast address is contiguous bit-wise maskable, similar to
   IPv4 addresses under Class-less Interdomain Routing [CIDR].

   There are several forms of unicast address assignment in IPv6,
   including the global provider based unicast address, the geographic
   based unicast address, the NSAP address, the IPX hierarchical
   address, the site-local-use address, the link-local-use address, and
   the IPv4-capable host address.  Additional address types can be
   defined in the future.

   IPv6 nodes may have considerable or little knowledge of the internal
   structure of the IPv6 address, depending on the role the node plays
   (for instance, host versus router).  At a minimum, a node may
   consider that unicast addresses (including its own) have no internal
   structure:

    |                           128 bits                              |
    +-----------------------------------------------------------------+
    |                          node address                           |
    +-----------------------------------------------------------------+


   A slightly sophisticated host (but still rather simple) may
   additionally be aware of subnet prefix(es) for the link(s) it is
   attached to, where different addresses may have different values for
   n:

    |                         n bits                 |   128-n bits   |
    +------------------------------------------------+----------------+
    |                   subnet prefix                | interface ID   |
    +------------------------------------------------+----------------+


   Still more sophisticated hosts may be aware of other hierarchical
   boundaries in the unicast address.  Though a very simple router may
   have no knowledge of the internal structure of IPv6 unicast




 
RFC 1884              IPv6 Addressing Architecture         December 1995


   addresses, routers will more generally have knowledge of one or more
   of the hierarchical boundaries for the operation of routing