Ross Vandegrift <ross@kallisti.us> writes:

Ahhh, thanks - that is the only thing I have ever seen that gives any reason for the /64 prefix. Sadly, the document contains no compelling technical reasons for it - looks like it's done just so things are easy when generating interface IDs from ethernet addresses.

One good guess for the rationale is to preserve the possibility of splitting the host and network identifier namespaces, like XNS and some others do. Should it be deemed the way forward, the current policy makes it slightly more of a possibility to decouple them after the fact.