IPv6 is Classless

Over the history of the IPv6 protocol, several classful addressing models have been proposed. The most notable example recommended Top-Level Aggregation (TLA) and Next-Level Aggregation (NLA) Identifiers , but was obsoleted by , leaving a single remnant of classful addressing in IPv6: a rigid network interface identifier boundary at /64. This document removes the fixed position of that boundary for interface addressing. Recent proposed changes to the IP Version 6 Addressing Architecture specification have caused controversy. While link prefixes of varied lengths, e.g. /127, /126, /124, /120, ... /64 have been successfully deployed for many years, glaring mismatches between a formal specification and long-standing field deployment practices are never wise, not least because of the strong risk of mis-implementation, which can easily result in serious operational problems. This document also clarifies that IPv6 routing subnets may be of any length up to 128.

It is assumed that the reader understands the history of classful addressing in IPv4 and why it was abolished . Of course, the acute need to conserve address space that forced the adoption of classless addressing for IPv4 does not apply to IPv6, but the arguments for operational flexibility in address assignment remain compelling. It is also assumed that the reader understands IPv6 , the IP Version 6 Addressing Architecture , the proposed changes to RFC4291 and RFC2464 , and the IETF recommendation for the generation of stable Interface Identifiers . For host computers on local area networks, generation of interface identifiers is no longer necessarily bound to layer 2 addresses (MACs) . Therefore their length, previously fixed at 64 bits , is in fact a variably-sized parameter as explicitly acknowledged in Section 5.5.3(d) of which states: Note that a future revision of the address architecture [RFC4291] and a future link-type-specific document, which will still be consistent with each other, could potentially allow for an interface identifier of length other than the value defined in the current documents. Thus, an implementation should not assume a particular constant. Rather, it should expect any lengths of interface identifiers.

IPv6 unicast interfaces may use any subnet length up to 128 except for situations where an Internet Standard document may impose a particular length, for example Stateless Address Autoconfiguration (SLAAC) , or Using 127-Bit IPv6 Prefixes on Inter-Router Links . Additionally, this document clarifies that a node or router MUST support routing of any valid network prefix length, even if SLAAC or other standards are in use, because routing could choose to differentiate at a different granularity than is used by any such automated link local address configuration tools.

For historical reasons, when a prefix is needed on a link, barring other considerations, a /64 is recommended . The length of the Interface Identifier in Stateless Address Autoconfiguration is a parameter; its length SHOULD be sufficient for effective randomization for privacy reasons. For example, a /48 might be sufficient. But operationally we recommend, barring strong considerations to the contrary, using 64-bits for SLAAC in order not to discover bugs where 64 was hard-coded, and to favor portability of devices and operating systems. Nonetheless, there is no reason in theory why an IPv6 node should not operate with different interface identfier lengths on different physical interfaces. Thus, a correct implementation of SLAAC must in fact allow for any prefix length, with the value being a parameter per interface. For instance, the Interface Identifier length in the recommended (see ) algorithm for selecting stable interface identifiers is a parameter, rather than a hardcoded value.

Assuming that nodes employ unpredictable interface identifiers , the subnet size may have an impact on some security and privacy properties of a network. Namely, the smaller the subnet size, the more feasible it becomes to perform IPv6 address scans . For some specific subnets, such as point to point links, this may be less of an issue. On the other hand, we assume that a number of IPv6 implementations fail to enforce limits on the size of some of the data structures they employ for communicating with neighboring nodes, such as the Neighbor Cache. In such cases, the use of smaller subnets forces an operational limit on such data structures, thus helping mitigate some pathological behaviors (such as Neighbor Cache Exhaustion attacks).

This document has no IANA Considerations.

The original sketch was by Randy Bush, who was immediately aided and abetted by Brian Carpenter, Chris Morrow, Fernando Gont, Geoff Huston, Job Snijders, and Nick Hilliard.

The authors wish to thank .