On 8/19/08 1:36 PM, "Nathan Ward" <nanog@daork.net> wrote:
64 bits is not a magical boundary.
112 bits is widely recommended for linknets, for example.
64 bits is common, because of EUI-64 and friends. That's it. There is nothing, anywhere, that says that the first 64 bits is for routing.
Actually, there is text that says so: RFC4291: IPv6 Addressing Architecture, section 2.5.1 " For all unicast addresses, except those that start with the binary value 000, Interface IDs are required to be 64 bits long and to be constructed in Modified EUI-64 format." The fact that most implementations ignore this is a different story. In practice, many routers require the packet to go twice in the hardware if the prefix length is > 64 bits, so even though it is a total waste of space, it is not stupid to use /64 for point-to-point links and even for loopbacks! - Alain.