Thanks everybody for your help. I was using a Mac mini as an exercise as I had one to hand for a week. I had thought that I'd have to buy something that I could put two ethernet cards in to do the job properly when I replace the old UNIX box but then I hadn't realised that USB ethernet ports we're so cheap, so I'll probably end up using one on a Mac mini. It is working with a single port for testing, the tip about adding a second network port in Preferences so that I didn't need an ifconfig alias was very handy. Thanks, regards, Rob.