RFC 3587 (rfc3587) - Page 2 of 5

IPv6 Global Unicast Address Format

RFC 3587           IPv6 Global Unicast Address Format        August 2003


   The IETF has provided technical input to the RIRs (for example,
   [RFC 3177]), which the RIRs have taken into account when defining
   their address allocation policy.

   RFC 2374 was the definition of addresses for Format Prefix 001
   (2000::/3) which is formally made historic by this document.  Even
   though currently only 2000::/3 is being delegated by the IANA,
   implementations should not make any assumptions about 2000::/3 being
   special.  In the future, the IANA might be directed to delegate
   currently unassigned portions of the IPv6 address space for the
   purpose of Global Unicast as well.

   The Subnet Local Aggregator (SLA) field in RFC 2374 remains in
   function but with a different name in [ARCH].  Its new name is
   "subnet ID".

3.  Address Format

   The general format for IPv6 global unicast addresses as defined in
   "IP Version 6 Addressing Architecture" [ARCH] 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 subnet within the
   site, and the interface ID is as defined in section 2.5.1 of [ARCH].
   The global routing prefix is designed to be structured hierarchically
   by the RIRs and ISPs.  The subnet field is designed to be structured
   hierarchically by site administrators.

   [ARCH] also requires that all unicast addresses, except those that
   start with binary value 000, have Interface IDs that are 64 bits long
   and to be constructed in Modified EUI-64 format.  The format of
   global unicast address in this case is:

    |         n bits          | 64-n bits |       64 bits              |
    +-------------------------+-----------+----------------------------+
    | global routing prefix   | subnet ID |       interface ID         |
    +-------------------------+-----------+----------------------------+








Hinden, et al.               Informational