http://portforward.com/
Click on the "Routers" button and go to Thompson. There are two 501 so try them both (unless you know precisely wich one is yours).
Make sure you also have properly configured your firewall according to the official LW site.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-