Anyone contemplating using multiple routes with different netmasks on the same
physical interface under at least NetBSD -- but probably any 4.4BSD derivative
-- needs to be aware that this probably doesn't work.  Last time I tried this,
accidentally (by not specifying _any_ netmask for an alias), I found that some
time later -- I assume when the radix tree was next recomputed, thogh I don't
know what causes such recomputations -- the netmask for the first IP address
on the interface became the mask of the alias I'd added, which caused quite a
few problems.

I tried replacing the radix code with the latest version, on a suggestion from
the gated mailing list if I recall correctly, and found that it didn't help. 
This is one to watch out for; probably a bug in the 4.4 code, though the RFC
is not sufficiently specific about distinguishing logical and physical
interfaces (it makes no distinction) to make it clear that this behaviour is
illegal.

Like the problem with the multicast code that makes it impossible to speak
OSPF across PPP to a Cisco, this is one of the reasons 4.4BSD boxes don't make
fantastic routers at the moment.