From: ipv6 [mailto:ipv6-bounces@xxxxxxxx] On Behalf Of Lorenzo Colitti > Help he understand, then. There is widely-deployed code that assumes > that the interface ID is 64 and does not work on anything other than > 64 bit prefix lengths. Currently that code is correct on all unicast > space. If you change RFC 4291, won't that code be incorrect? This shows precisely why it is urgent to update RFC 4291, to correct that notion of a fixed IID, before it's too late to set things straight again. RFC 4291 describes any number of address prefixes that are not /64. For example: IPv6 unicast addresses are aggregatable with prefixes of arbitrary bit-length, similar to IPv4 addresses under Classless Inter-Domain Routing. Important point. Arbitrary length. That does not mean 64 bits. And | n bits | 128-n bits | +-------------------------------+---------------------------------+ | subnet prefix | interface ID | +-------------------------------+---------------------------------+ This next needs to be clarified/corrected, as it should only apply to the 2000::/3 space: All Global Unicast addresses other than those that start with binary 000 have a 64-bit interface ID field (i.e., n + m = 64), formatted as described in Section 2.5.1. Global Unicast addresses that start with binary 000 have no such constraint on the size or structure of the interface ID field. Bert