I need a starting point for writing a simple firewall in C#....maybe 10-20 lines?
When an application is running on a port I want to intercept incoming
traffic. Under set conditions (such as the originating IP being in a
certain range) the request is blocked, otherwise the request is
allowed to continue onto the target program.
Is there a way to do this using the .Net framework?
I am not looking for a commercial component.

Why do you want to do this in C#? There is already a variety of
software and code out on the internet to do exactly this, or that can
be adapted to easily do this. I do not know .NET or C#, however this
seems like you are trying to do something that has already been done a
number of times.

I want to integrate firewall capabilities into an application that is
written in C#.     If its possible it makes sense for the firewall to
be fully itegrated by writing it in the same language.