INTERNET PROTOCOL VERSION 6 ADDRESS SPACE

As defined in [RFC2373] the specific type of an IPv6 address is indicated
by the leading bits in the address.  The variable-length field comprising
these leading bits is called the Format Prefix (FP).

The current allocation of these prefixes is listed below.

    Allocation                            Prefix         Fraction of
                                          (binary)       Address Space
    -----------------------------------   --------       -------------
    Reserved                              0000 0000      1/256
    Unassigned                            0000 0001      1/256

    Reserved for NSAP Allocation          0000 001       1/128
    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     [RFC2374]

    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", the "loopback address", and the IPv6
     Addresses with Embedded IPv4 Addresses are assigned out
     of the 0000 0000 format prefix space and are defined in [RFC2373].

 (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.

IPv6 Unicast addresses are distinguished from multicast addresses by the
value of the high-order octet of the addresses: a value of 0xFF (binary
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.


References
----------

[RFC2373] Hinden, R., and S. Deering, "IP Version 6 Addressing
          Architecture", RFC 2373, Nokia, Cisco Systems,
          July 1998.

[RFC2374] Hinden, R., M. O'Dell, and S. Deering, "An Aggregatable Global
          Unicast Address Format", RFC 2374, Nokia, UUNET, Cisco Systems,
          July 1998.


[Last updated 17 August 1999]