I've been thinking about your problem, Freebird, and the main stumbling block is that you require a bridgewall i.e. a bridging level firewall. If you're on a budget I have not come across any free or low-cost examples of this type of software (Windows or Linux). Of course you could build a linux box with bridging enabled and manually configure it to act as a bridgewall. Check out this
link
IPCOP/Smoothwall or similarly packaged "firewall in a box" solutions will not do it as they usually NAT everything in sight.
Another alternative if you want to play with your broadband server is to actually setup the iptables rules and implement the filtering at the broadband server as opposed to having an additional box in between your users and server.
