Thanks very much to David and Bruce for your replies.

@ David: the modem is a Belkin ADSL2+ modem, which I believe is available only here in Australia. It was supplied to me by my ISP. (I took the path of least resistance on that!)

@ Bruce: your reply prompted me to look at the fine manual that came with the modem. There's a section in it called "Setting MAC address filtering" which seems to allow me to do the sort of thing you're suggesting. I really should've looked there first, shouldn't I?

Thanks again,

Leslie