Reserved IPv6 Subnet Anycast Addresses
RFC 2526

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Network Working Group                                         D. Johnson
Request for Comments: 2526                    Carnegie Mellon University
Category: Standards Track                                     S. Deering
                                                     Cisco Systems, Inc.
                                                              March 1999

                 Reserved IPv6 Subnet Anycast Addresses

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   The IP Version 6 addressing architecture defines an "anycast" address
   as an IPv6 address that is assigned to one or more network interfaces
   (typically belonging to different nodes), with the property that a
   packet sent to an anycast address is routed to the "nearest"
   interface having that address, according to the routing protocols'
   measure of distance.  This document defines a set of reserved anycast
   addresses within each subnet prefix, and lists the initial allocation
   of these reserved subnet anycast addresses.

1. Introduction

   IP Version 6 (IPv6) defines a new type of address, known as an
   "anycast" address, that allows a packet to be routed to one of a
   number of different nodes all responding to the same address [2, 3].
   The anycast address may be assigned to one or more network interfaces
   (typically on different nodes), with the network delivering each
   packet addressed to this address to the "nearest" interface based on
   the notion of "distance" determined by the routing protocols in use.

   The uses of anycast addresses are still evolving, but such addresses
   offer the potential for a number of important services [5, 6].  For
   example, an anycast address may be used to allow nodes to access one
   of a collection of servers providing a well-known service, without
   manual configuration in each node of the list of servers; or an

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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                     [Page 2]
RFC 2526         Reserved IPv6 Subnet Anycast Addresses       March 1999