Just thought I'd chip in my two cents on all of this. Pardon me if
I'm not totally coherent... I've only had a few hours of sleep in the
last few days.
It seems to me that the best solution is a unified CIDR type (using
the proper terminology) which can represent hosts, networks, and/or
netmasks in a variety of address families, notably IPv4 and IPv6.
Perhaps I don't fully understand what capabilities we have with
atttypmod, but could we use it to "subtype" the CIDR type? Then
various operations, including input/output, queries, and indexing,
could act accordingly.
-Brandon :)