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RFC 3587

IPv6 Global Unicast Address Format

Pages: 5
Obsoletes:  2374

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


This document obsoletes RFC 2374, "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 RFC 2374 and the TLA/NLA structure historic.

1. Introduction

RFC 2374, "An IPv6 Aggregatable Global Unicast Address Format", defined an IPv6 address allocation structure that includes TLA and NLA. This document replaces RFC 2374, and makes RFC 2374 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The TLA/NLA scheme has been replaced by a coordinated allocation policy defined by the Regional Internet Registries (RIRs) [IPV6RIR].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.
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   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.

   RFC 2374 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 RFC 2374 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 | +-------------------------+-----------+----------------------------+
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   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 RFC 3177 is:

    | 3 |     45 bits         |  16 bits  |       64 bits              |
    |001|global 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", RFC 3513, April 2003. [IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.

5.2. Informative References

[IPV6RIR] APNIC, ARIN, RIPE NCC, "IPv6 Address Allocation and Assignment Policy", Document ID: ripe-267,, January 22, 2003. [RFC3177] IAB/IESG, "Recommendations on IPv6 Address Allocations to Sites", RFC 3177, September 2001.

6. Security Considerations

IPv6 addressing documents do not have any direct impact on Internet infrastructure security.
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7. Authors' Addresses

Robert M. Hinden Nokia 313 Fairchild Drive Mountain View, CA USA EMail: 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:
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8. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society.