RFC 2526 (rfc2526) - Page 2 of 7

Reserved IPv6 Subnet Anycast Addresses

RFC 2526         Reserved IPv6 Subnet Anycast Addresses       March 1999


   anycast address may be used in a source route to force routing
   through a specific internet service provider, without limiting
   routing to a single specific router providing access to that ISP.

   IPv6 defines a required Subnet-Router anycast address [3] for all
   routers within a subnet prefix, and allows additional anycast
   addresses to be taken from the unicast address space.  This document
   defines an additional set of reserved anycast addresses within each
   subnet prefix, and lists the initial allocation of these reserved
   subnet anycast addresses.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

2. Format of Reserved Subnet Anycast Addresses

   Within each subnet, the highest 128 interface identifier values are
   reserved for assignment as subnet anycast addresses.

   The construction of a reserved subnet anycast address depends on the
   type of IPv6 addresses used within the subnet, as indicated by the
   format prefix in the addresses.  In particular, for IPv6 address
   types required to have 64-bit interface identifiers in EUI-64 format,
   the universal/local bit MUST be set to 0 (local) in all reserved
   subnet anycast addresses, to indicate that the interface identifier
   in the address is not globally unique.  IPv6 addresses of this type
   are currently specified to be those having format prefixes 001
   through 111, except for Multicast Addresses (1111 1111) [3].

   Specifically, for IPv6 address types required to have to have 64-bit
   interface identifiers in EUI-64 format, these reserved subnet anycast
   addresses are constructed as follows:

   |              64 bits            |      57 bits     |   7 bits   |
   +---------------------------------+------------------+------------+
   |           subnet prefix         | 1111110111...111 | anycast ID |
   +---------------------------------+------------------+------------+
                                     |   interface identifier field  |

   For other IPv6 address types (that is, with format prefixes other
   than those listed above), the interface identifier is not in EUI-64
   format and may be other than 64 bits in length; these reserved subnet
   anycast addresses for such address types are constructed as follows:







Johnson & Deering           Standards Track