ipfwadm -F -a m -S 10.100.100.2/32 -D 0.0.0.0/0
but not under 2.1.22.
This works with the new kernels instead:
ipfwadm -F -a m -S 10.100.100.0/24 -D 0.0.0.0/0
I tested only a telnet connection, so I can't confirm
whether the same applies to ftp, but I've no reasons to
doubt it does.
Ciao