as it's currently worded, draft-ietf-6man-rfc4291bis seems to prohibit the implementation of any interface netmask != /64: > However, the Interface ID of > all unicast addresses, except those that start with the binary value > 000, is required to be 64 bits long. This has substantial operational consequences in the ipv6 world because if it's implemented as stated, it will cause production ipv6 networks to break. The ipv6 operational community may have opinions on the wisdom of mandating new behaviour which would cause their networks to fall over, so it would probably be a good idea to notify v6ops@ietf about the existence of this draft so that the folks over there get a look-in before a consensus call is made. As far as I can tell, this notification never happened. Nick