I have Internet ---- modem ---- m0n0 --- LAN with several PCs

I can ping a.b.c.d from any PC, however I can only ping a.b.c.d from
one PC at a time.

If I ping from pc X then pc Y will not be able to ping a.b.c.d, though
it can ping other hosts on the internet.
pc X can have 2 windows ping away fine to the same host.

Is this a m0n0 restriction? NAT problem ?

managed to replicate this on 2 different networks with 2 different
isps, so it seems to be down to m0n0.

sai