IPv6 is Classless

Over the history of IPv6, various classful address models have been proposed, with the most notable being Top-Level Aggregation (TLA) and Next-Level Aggregation (NLA) Identifiers; see, for example, . They have all proved to be mistakes. For example, TLA and NLA were obsoleted by . The last remnant of classful addressing is a rigid network / interface identifier boundary at /64. This document removes that boundary as far as routing and addressing are concerned. Some confusion has been caused by the IP Version 6 Addressing Architecture, , and the proposed changes in with respect to the minimum subnet size. Meanwhile, link prefixes of varied lengths, /127, /126, /124, /120, ... /64 have been successfully deployed for many years. Having the formal specification be unclear risks potential mis-implementation by the naïve, which could result in operational disasters.

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 allocation 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 recent recommendations for the generation of stable Interface Identifiers . An important recent IPv6 development was that, for host computers on local area networks, the way in which interface identifiers were formed was no longer bound to layer 2 addresses (MACs) . Therefore their length, previously fixed at 64 bits , is in fact a variably-sized parameter as stated in .

To state it simply, IPv6 unicast subnetting is based on prefixes of any valid length up to 128 except for links where an Internet Standard that has nothing to do with routing may impose a particular length. Examples are Stateless Address AutoConfiguration (SLAAC) , or Using 127-Bit IPv6 Prefixes on Inter-Router Links . Nodes must always support routing on 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. None the less, 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.

Assumming 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 draft was by Randy Bush, who was immediately aided and abetted by Brian Carpenter, Chris Morrow, Fernando Gont, Geoff Huston, Job Snijders, [ your name here ].

The authors wish to thank .