Well you didn't tell us how all these computers connect to the modem. I assume a router, and at least one switch (besides that built into the router) or hub.
WallWatcher currently supports collecting and analyzing logs from 100+ routers. If your router generates logs, this may be what you need - but it will take some studying to set up properly. There are many other products for network traffic monitoring, including hardware devices, most are not free and many are much more complex, and informative. Entering
monitor home network traffic in Google reveals some options.