Bittorrent Port on ADSL Modem

[gbmp3]

I have a NetComm ADSL Modem Plus 4 and can connect easily to the internet.
But ....
I wish to use Bittorrent and the default bittorrent port is 6881 but my system tells me that this comes up with a NAT error.
Any ideas what the correct bittorrent ports are ????