Do you mean LW works in OS 9? Your test which failed suggests you can't connect with LW in OSX b/c the port is blocked. Now we need to find out why the port is blocked. I would tend to suggest it's your D-Link router if it's not your isp. (By the way did you do the same test in OS9?)

So I believe you may need to forward a port for LW. If you already forwarded a port for your G4 then you'll need to choose a different port for your G5. 2 computers cannot use the same port when port forwarding under a nat device. If you haven't previously forwarded a port for your router then this is what you'll need to do:

1. Set up a Static IP (very important else forwarding a port won't work!)

2. Forward a port. By default LW uses port 6346 but you can choose another like port 6342 or something. You'll probably have a D-Link manual which shows you how. If not then you can get hints from here: http://www.portforward.com/routers.htm Bottom-left side of page! (mind you these instructions are for windows.)

3. Within LW: Manual port forward instructions & sample image & 2 posts after that.