INTERNET PROTOCOL VERSION 6 ANYCAST ADDRESSES

IPv6 anycast addresses are defined in "IP Version 6 Addressing
Architecture" [RFC2373].  

Anycast addresses are taken from the unicast address space, and are not
syntactically distinguishable from unicast addresses.


Subnet-Router Anycast Address
-----------------------------

[RFC2373] defines the Subnet-Router Anycast Address.  Its format is as
follows:

   |                         n bits                 |   128-n bits   |
   +------------------------------------------------+----------------+
   |                   subnet prefix                | 00000000000000 |
   +------------------------------------------------+----------------+

The "subnet prefix" in an anycast address is the prefix which identifies
a specific link.  This anycast address is syntactically the same as a
unicast address for an interface on the link with the interface
identifier set to zero.

Packets sent to the Subnet-Router anycast address will be delivered to
one router on the subnet.  All routers are required to support the
Subnet-Router anycast addresses for the subnets on which they have
interfaces.


Reserved Subnet Anycast Addresses 
---------------------------------

[RFC2526] further defines a set of reserved subnet anycast addresses.
These are the highest 128 interface identifier values, within each
subnet.  The syntax and rules for assigning new IPv6 reserved subnet
anycast addresses are defined in [RFC2526].  A 7-bit reserved subnet
anycast identifier is used to create each each reserved subnet anycast
address.

The current IPv6 reserved subnet anycast identifiers are listed below.

     Decimal   Hexadecimal     Description
     -------   -----------     -----------
      127       0x7F           Reserved
      126       0x7E           Mobile IPv6 Home-Agents anycast [JOHNSON]
      0         0x00           Reserved

All IPv6 reserved subnet anycast identifiers not listed above are
reserved.


References
----------

[JOHNSON] David B. Johnson and Charles Perkins, "Mobility Support in
          IPv6", Work in Progress, IP Mobility w.g.

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

[RFC2526] Johnson, D., S. Deering, "Reserved IPv6 Subnet Anycast
          Addresses", RFC2526, Carnegie Mellon University, Cisco Systems,
          March 1999


[Last updated 19 August 1999]