I am a newbie to freebsd and want to ask a few questions:

there's a firewall running freebsd 4.2 which act as gateway and nat server
for my intranet (192.168.1.0 xl0)  and a PPPoE internet connection(tun0)

suppose i want to make remote X possible (from internet back to one of the
machine in the intranet, say 192.168.1.101), then what should i do with the
ipfw?

If you can, try comp.unix.bsd.freebsd.misc instead of this group.

X uses a bunch of different ports starting from 6000.  However, it is
terribly insecure to run over trusted networks; try using ssh (built
into FreeBSD these days) to tunnel it instead.