Problem disconnecting from internet

We installed a Windows 2k server with two nic’s. One for the internal network, and one for the internet.
The connection to the ISP is done by a Cisco 800 router over an ISDN-line.
We were able to connect to the ISP and the line disconnected after 5 minutes of inactivity. After we installed DNS-services, we could still connect to the ISP, but the line did not disconnect after 5 minutes. It seems DNS is sending a broadcast over the internet resetting the inactivity timer.
How can I prevent DNSto send these broadcasts to the router or is there an other solution for our problem?
You can contact me at m.smit02@chello.nl