Network Working Group D. Johnson
Request for Comments: 2526 Carnegie Mellon University
Category: Standards Track S. Deering
Cisco Systems, Inc.
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 (C) The Internet Society (1999). All Rights Reserved.
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.
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
Johnson & Deering Standards Track [Page 1]
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  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 .
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) .
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