On Mar 21, 2012, at 10:51 PM, Brian E Carpenter wrote: > In other words if the IETF doesn't define the zone index, every > implementor will have to do so anyway. Also, read the last clause > carefully: it says the stack MUST allow OPTIONAL use of the zone > index internally. Implementors generally *do* have some internal form of a zone index, and it doesn't look at all like what the RFC describes. It is a machine address or a lookup index of some kind for a table that is associated with an interface. Sometimes, part of it is a card identifier. >> From MIF's perspective, if the same prefix is placed on multiple interfaces, > > The prefix fe80::/10 is automatically on every interface. That's the > only case where I'm certain we need a zone index in practice, but the > definition isn't limited to that case. The use of something to get me to the interface table in question isn't what I am questioning. It's the use of that particular something...