Network Working Group R. Hinden
Request for Comments: 3587 Nokia
Obsoletes: 2374 S. Deering
Category: Informational Cisco
E. Nordmark
Sun
August 2003

IPv6 Global Unicast Address Format

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

This document obsoletes RFC2374, "An IPv6 Aggregatable Global
Unicast Address Format". It defined an IPv6 address allocation
strUCture that includes Top Level Aggregator (TLA) and Next Level
Aggregator (NLA). This document makes RFC2374 and the TLA/NLA
structure historic.

1. Introduction

RFC2374, "An IPv6 Aggregatable Global Unicast Address Format",
defined an IPv6 address allocation structure that includes TLA and
NLA. This document replaces RFC2374, and makes RFC2374 and the
TLA/NLA structure historic.

2. TLA/NLA Made Historic

The TLA/NLA scheme has been replaced by a coordinated allocation
policy defined by the Regional Internet Registries (RIRs) [IPV6RIR].

Part of the motivation for obsoleting the TLA/NLA structure is
technical; for instance, there is concern that TLA/NLA is not the
technically best approach at this stage of the deployment of IPv6.
Moreover, the allocation of IPv6 addresses is related to policy and
to the stewardship of the IP address space and routing table size,
which the RIRs have been managing for IPv4. It is likely that the
RIRs' policy will evolve as IPv6 deployment proceeds.

The IETF has provided technical input to the RIRs (for example,
[RFC3177]), which the RIRs have taken into account when defining
their address allocation policy.

RFC2374 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 RFC2374 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
+-------------------------+-----------+----------------------------+

where the routing prefix is a value assigned to identify a site (a
cluster of subnets/links), the subnet ID is an identifier of a subnet
within the site, and the interface ID is a modified EUI-64 format as
defined in [ARCH].

An example of the resulting format of global unicast address under
the 2000::/3 prefix that is currently being delegated by the IANA and
consistent with the recommendations in RFC3177 is:

3 45 bits 16 bits 64 bits
+---+---------------------+-----------+----------------------------+
001global routing prefix subnet ID interface ID
+---+---------------------+-----------+----------------------------+

4. Acknowledgments

The authors would like to eXPress our thanks to Alain Durand, Brian
Carpenter, Fred Templin, Julian Sellers, Jun-ichiro Itojun Hagino,
Margaret Wasserman, Michel Py, Pekka Savola, Tatuya Jinmei, and
Thomas Narten for their review and constructive comments.

5. References

5.1. Normative References

[ARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC3513, April 2003.

[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC2460, December 1998.

5.2. Informative References

[IPV6RIR] APNIC, ARIN, RIPE NCC, "IPv6 Address Allocation and
Assignment Policy", Document ID: ripe-267,
http://www.ripe.net/ripe/docs/ipv6policy.Html, January 22,
2003.

[RFC3177] IAB/IESG, "Recommendations on IPv6 Address Allocations to
Sites", RFC3177, September 2001.

6. Security Considerations

IPv6 addressing documents do not have any direct impact on Internet
infrastructure security.

7. Authors' Addresses

Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA
USA

EMail: bob.hinden@nokia.com

Stephen E. Deering
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134-1706
USA

Erik Nordmark
Sun Microsystems Laboratories
180, avenue de l'Europe
38334 SAINT ISMIER Cedex
France

EMail: erik.nordmark@sun.com

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