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6man Working Group The length of IP prefixes is an information used by forwarding and routing processes is policy-based. As such, no maximum length must be assumed by design. Discussions on the 64-bit boundary in IPv6 addressing revealed a need for a clear recommendation on which bits must be used by forwarding decision-making processes. This document sketches a recommendation to be followed by forwarding and routing designs with regards to the prefix length. The aim is to avoid hard-coded routing and forwarding designs that exclude some IP prefix lengths. 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.

Recent discussions on the 64-bit boundary in IPv6 addressing () revealed a need for a clear recommendation on which bits must be used by forwarding decision-making processes. A detailed analysis of the 64-bit boundary in IPv6 addressing, and the implication for end-site prefix assignment, is documented in . No recommendation is included in . It is fundamental to not link routing and forwarding to the IPv6 prefix/address semantics . This document includes a recommendation for that aim. Forwarding decisions made by routers primarily rely upon a longest prefix-match algorithm. Like in IPv4, the IPv6 prefix-match algorithms involve one critical operation which is the comparison of a destination address with a prefix present in a routing table (e.g., compare the 2001:db8::1 address with the 2001:db8::/64 prefix). The recommendation of this document is to be followed by that critical operation. It is important that the compare operation be a bit-wise comparison, and not a byte-wise comparison.

Forwarding decision-making processes MUST NOT restrict by design the length of IPv6 prefixes. In particular, forwarding processes MUST be designed to process prefixes of any length up to /128, by increments of 1. Obviously, 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. This recommendation does not conflict with the 64-bit boundary for IPv6 stateless address autoconfiguration (SLAAC) . Some lookup algorithm implementations (find the prefix matching a given destination address) may be affected by this recommendation, even more so for IPv6 than IPv4. The performance of some implementations may be degraded when prefix lengths are longer than /64.

This document does not require any action from IANA.

This document does not introduce security issues in addition to what is discussed in .

Thanks to Eric Vyncke and Christian Jacquenet for their comments. Special thanks to Randy Bush and Brian Carpenter for their support.