IPv6 Prefix Length Recommendation for Forwarding

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. However, such a recommendation was out of scope for that document. Although Section 2.5 of states "IPv6 unicast addresses are aggregatable with prefixes of arbitrary bit-length, similar to IPv4 addresses under Classless Inter-Domain Routing" (CIDR, ), there is still a misinterpretation that IPv6 prefixes can be either /127 () or any length up to /64. This misinterpretation is mainly induced by the 64-bit boundary in IPv6 addressing. As discussed in , "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 , 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. It is fundamental not to link routing and forwarding to the IPv6 prefix/address semantics . This document includes a recommendation for that aim. 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 () is that IPv6 forwarding must follow the longest-match-first rule, regardless of prefix length, unless some overriding policy is configured. This recommendation does not conflict with the 64-bit boundary for some IPv6 stateless address autoconfiguration (SLAAC, ) based schemes such as . Indeed, 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 ). A historical background of CIDR is documented in and Section 2 of .

IPv6 implementations MUST conform to the rules specified in Section 5.1 of . 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.