Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 7608 France Telecom
BCP: 198 A. Petrescu
Category: Best Current Practice CEA, LIST
ISSN: 2070-1721 F. Baker
IPv6 Prefix Length Recommendation for Forwarding
IPv6 prefix length, as in IPv4, is a parameter conveyed and used in
IPv6 routing and forwarding processes in accordance with the
Classless Inter-domain Routing (CIDR) architecture. The length of an
IPv6 prefix may be any number from zero to 128, although subnets
using stateless address autoconfiguration (SLAAC) for address
allocation conventionally use a /64 prefix. Hardware and software
implementations of routing and forwarding should therefore impose no
rules on prefix length, but implement longest-match-first on prefixes
of any valid length.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Boucadair, et al. Best Current Practice [Page 1]
RFC 7608 July 2015
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Recommendation . . . . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 4
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Normative References . . . . . . . . . . . . . . . . . . 4
4.2. Informative References . . . . . . . . . . . . . . . . . 4
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
Discussions on the 64-bit boundary in IPv6 addressing ([RFC7421])
revealed a need for a clear recommendation on which bits must be used
by forwarding decision-making processes. However, such a
recommendation was out of scope for that document.
Although Section 2.5 of [RFC4291] states "IPv6 unicast addresses are
aggregatable with prefixes of arbitrary bit-length, similar to IPv4
addresses under Classless Inter-Domain Routing" (CIDR, [RFC4632]),
there is still a misinterpretation that IPv6 prefixes can be either
/127 ([RFC6164]) or any length up to /64. This misinterpretation is
mainly induced by the 64-bit boundary in IPv6 addressing.
As discussed in [RFC7421], "the notion of a /64 boundary in the
address was introduced after the initial design of IPv6, following a
period when it was expected to be at /80". This evolution of the
IPv6 addressing architecture, resulting in [RFC4291], and followed
with the addition of /127 prefixes for point-to-point links, clearly
demonstrates the intent for future IPv6 developments to have the
flexibility to change this part of the architecture when justified.
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RFC 7608 July 2015
It is fundamental not to link routing and forwarding to the IPv6
prefix/address semantics [RFC4291]. This document includes a
recommendation in order to support that goal.
Forwarding decisions rely on the longest-match-first algorithm, which
stipulates that, given a choice between two prefixes in the
Forwarding Information Base (FIB) of different length that match the
destination address in each bit up to their respective lengths, the
longer prefix is used. This document's recommendation (Section 2) is
that IPv6 forwarding must follow the longest-match-first rule,
regardless of prefix length, unless some overriding policy is
This recommendation does not conflict with the 64-bit boundary for
some schemes that based on IPv6 stateless address autoconfiguration
(SLAAC) [RFC4862], such as [RFC2464]. Indeed, [RFC7421] clarifies
this is only a parameter in the SLAAC process, and other longer
prefix lengths are in operational use (e.g., either manually
configured or based upon DHCPv6 [RFC3315]).
A historical background of CIDR is documented in [RFC1380] and
Section 2 of [RFC4632].
1.1. Requirements Language
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 [RFC2119].
IPv6 implementations MUST conform to the rules specified in
Section 5.1 of [RFC4632].
Decision-making processes for forwarding MUST NOT restrict the length
of IPv6 prefixes by design. In particular, forwarding processes MUST
be designed to process prefixes of any length up to /128, by
increments of 1.
Policies can be enforced to restrict the length of IP prefixes
advertised within a given domain or in a given interconnection link.
These policies are deployment specific and/or driven by
administrative (interconnection) considerations.
Boucadair, et al. Best Current Practice [Page 3]
RFC 7608 July 2015
3. Security Considerations
This document does not introduce security issues in addition to what
is discussed in [RFC4291].
IPv6 security issues, including operational ones, are discussed in
[RFC4942] and [OPSEC-v6].
4.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
4.2. Informative References
[OPSEC-v6] Chittimaneni, K., Kaeo, M., and E. Vyncke, "Operational
Security Considerations for IPv6 Networks", Work in
Progress, draft-ietf-opsec-v6-06, March 2015.
[RFC1380] Gross, P. and P. Almquist, "IESG Deliberations on Routing
and Addressing", RFC 1380, DOI 10.17487/RFC1380, November
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
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RFC 7608 July 2015
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
Co-existence Security Considerations", RFC 4942,
DOI 10.17487/RFC4942, September 2007,
[RFC6164] Kohno, M., Nitzan, B., Bush, R., Matsuzaki, Y., Colitti,
L., and T. Narten, "Using 127-Bit IPv6 Prefixes on Inter-
Router Links", RFC 6164, DOI 10.17487/RFC6164, April 2011,
[RFC7421] Carpenter, B., Ed., Chown, T., Gont, F., Jiang, S.,
Petrescu, A., and A. Yourtchenko, "Analysis of the 64-bit
Boundary in IPv6 Addressing", RFC 7421,
DOI 10.17487/RFC7421, January 2015,
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Thanks to Eric Vyncke, Christian Jacquenet, Brian Carpenter, Fernando
Gont, Tatuya Jinmei, Lorenzo Colitti, Ross Chandler, David Farmer,
David Black, and Barry Leiba for their contributions and comments.
Special thanks to Randy Bush for his support.
Gif-sur-Yvette, Ile-de-France 91190
Santa Barbara, California 93117
Boucadair, et al. Best Current Practice [Page 6]
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