...

there are valid cases where a (globally unique) AS# is required which does not show up in the DFZ. (and, of course, there are places where a private AS# would do just as well). Hmmm. I'm curious: could you name a few? The things that I could imagine are "private peerings somewhere", which "should" work using private AS#s just fine.

BGP within _one_ administrative domain can (most probably in most cases) use private AS#s, and indeed we are using them in that situation. However when the peerings have to span policy/operations-coordination/ ISP boundaries, then private number space becomes unmanageable. Wilfried.