Subnet declaration for each interface

[Ted Lemon]

This code is really complicated, because of the different systems it has to 
support.   I think the best way to grok it is to run it on a unix machine 
and walk through it.   I'd explain it for you, but TBH I don't remember off 
the top of my head.   :'}   I *think* tmp->shared_net gets initialized in 
an interface setup hook or something like that, but I'm not sure.