I've got a Cable Modem Connected to my AirPort Extreme Base Station that I access with my Powermac through an Ethernet Hub. I also access it wirelessly with my powerbook. I also have a second APExtreme Base bridged to it for my TIVO and Cable Box.
I have the Airport Base set to only allow access to my Powerbook and bridged AP Base MAC addresses. So the only things I have turned on are access control and WEP for security. I tried creating a closed network but it disrupted my bridge. So my Net still appears to anyone with a 802.11g. (I haven't tried Radius yet, I don't know if it will allow bridging.)
Lately, with all my computers off and no activity on the bridged Airpot, I've noticed a lot of activity on my Cable Modem and main Base Station. How can I tell if someone else is accessing my connection, or if it's just the Cable modem pinging Comcast?