Microsoft systems are notorious for sending broadcast messages. These messages tend to trick ISDN routers into connecting to the Internet. You are right that the proper solution is to implement filters; however, I do not know how to configure your specific router. I checked Netgear's site for you, but could not find online documentation. I hate to do this, but I have to recommend that you refer to the manual or call their technical support for more information.
This was first published in July 2001