you have to sniff TCP packets, but I don't know how to do this in C#. try searching on CodeProject: I think there is something useful

You don't know much about network traffic, do you? Sniffers only look at what's passing by, they don't intercept and block anything... What your looking for is a router! Unless your machine is physically in the path of the network packets, meaning that ALL network traffic on that segment passes THROUGH your machine (not aorund or passes by it), there's nothing you can do to block anything. BTW, this is already implemented in the form of Windows Routing and Remote Access... RageInTheMachine9532 "...a pungent, ghastly, stinky piece of cheese!" -- The Roaming Gnome