Reserved for IPX Allocation           0000 010       1/128

    Unassigned                            0000 011       1/128
    Unassigned                            0000 1         1/32
    Unassigned                            0001           1/16

    Aggregatable Global Unicast Addresses 001            1/8
    Unassigned                            010            1/8
    Unassigned                            011            1/8
    Unassigned                            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 Unicast Addresses          1111 1110 10   1/1024
    Site-Local Unicast Addresses          1111 1110 11   1/1024

    Multicast Addresses                   1111 1111      1/256

    Notes:

      (1) The "unspecified address" (see section 2.5.2), the loopback
          address (see section 2.5.3), and the IPv6 Addresses with
          Embedded IPv4 Addresses (see section 2.5.4), are assigned out
          of the 0000 0000 format prefix space.








 
RFC 2373              IPv6 Addressing Architecture             July 1998


      (2) The format prefixes 001 through 111, except for Multicast
          Addresses (1111 1111), are all required to have to have 64-bit
          interface identifiers in EUI-64 format.  See section 2.5.1 for
          definitions.

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

2.5 Unicast Addresses

   IPv6 unicast addresses are aggregatable with contiguous bit-wise
   masks similar to IPv4 addresses under Class-less Interdomain Routing
   [CIDR].

   There are several forms of unicast address assignment in IPv6,
   including the global aggregatable global unicast address, the NSAP
   address, the IPX hierarchical address, the site-local address, the
   link-local 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:




 
RFC 2373              IPv6 Addressing Architecture             July 1998


   |                         n bits                 |   128-n bits   |
   +------------------------------------------------+----------------+