IPv6 Global Unicast Address Format
How a worldwide IPv6 address splits into routing prefix, subnet ID and interface ID, after the TLA/NLA scheme was retired.
Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global Unicast Address Format", RFC 3587, DOI 10.17487/RFC3587, August 2003, <https://www.rfc-editor.org/info/rfc3587>.
@misc{rfc3587,
series = {Request for Comments},
number = 3587,
howpublished = {RFC 3587},
publisher = {RFC Editor},
doi = {10.17487/RFC3587},
url = {https://www.rfc-editor.org/info/rfc3587},
author = {R. Hinden and S. Deering and E. Nordmark},
title = {{IPv6 Global Unicast Address Format}},
pagetotal = 5,
year = 2003,
month = aug,
} RFC text
Contents
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 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The TLA/NLA scheme has been replaced by a coordinated allocation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.
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 |
+-------------------------+-----------+----------------------------+
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,
http://www.ripe.net/ripe/docs/ipv6policy.html, 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.
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
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
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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
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followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1Introduction
RFC 2374, "An IPv6 Aggregatable Global Unicast Address Format", defined an IPv6 allocation structure that includes the Top Level Aggregator (TLA) and Next Level Aggregator (NLA). RFC 3587 replaces RFC 2374 and makes both it and the TLA/NLA structure historic.
An older plan for handing out worldwide addresses is being retired, and this memo says what comes next.
§2Why TLA/NLA Was Made Historic
The TLA/NLA scheme has been replaced by a coordinated allocation policy defined by the Regional Internet Registries (RIRs). Part of the motivation is technical: there is concern that TLA/NLA is not the technically best approach at this stage of IPv6 deployment. The rest is that allocation relates to policy and to stewardship of the address space and routing table size, which the RIRs have managed for IPv4.
RFC 2374 defined addresses for Format Prefix 001 (2000::/3). Although only 2000::/3 is currently being delegated by the IANA, implementations should not assume it is special, because the IANA might later delegate other unassigned portions for global unicast.
The Subnet Local Aggregator (SLA) field remains in function but is renamed subnet ID in the addressing architecture document.
Instead of one fixed plan, the groups that hand out addresses now set the rules. The room-number part of the address stays but gets a new name.
§3The Address Format
A global unicast address has a global routing prefix of n bits, a subnet ID of m bits and an interface ID of 128-n-m bits. The prefix is assigned to a site (a cluster of subnets or links) and is designed to be structured hierarchically by the RIRs and ISPs. The subnet ID identifies a subnet within the site and is structured by site administrators.
The architecture also requires that all unicast addresses except those starting with binary 000 have 64-bit interface IDs in Modified EUI-64 format. The layout is then n bits of prefix, 64-n bits of subnet ID and 64 bits of interface ID.
Under 2000::/3, consistent with the recommendations in RFC 3177, the example layout is 3 bits (001), a 45-bit global routing prefix, a 16-bit subnet ID and a 64-bit interface ID.
The first part says which site you are at, the middle says which network inside the site, and the last part says which device. For the example, the last part is 64 bits long.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
FORMAT-PREFIX | Format prefix | 3 bits | 001 | Binary 001. |
ROUTING-PREFIX | Global routing prefix | 45 bits | Assigned to a site. | |
SUBNET-ID | Subnet ID | 2 bytes | Identifies a subnet within the site. | |
INTERFACE-ID | Interface ID | 8 bytes | Modified EUI-64 format. |
§4Security Considerations
The RFC says only that IPv6 addressing documents do not have any direct impact on Internet infrastructure security.
Choosing how addresses are split up does not by itself make the Internet less safe.