here is my scenario:

we are developing a game. a multiplayer game. player A wants to host.
Player A has a router or is part of a greater network and so has no "direct" wire to the net. Player A is a gamer and does not know about ports. so he starts the server and wants his friends to join him. but they can't because they cannot connect, because the router is blocking the UDP port the game uses. player A blames the game and spams our support forum.

to prevent this, i wanted to include a small port checker utility that alerts the user if udp port xxxxx is closed, so he has to open it first before others can play with him.

i found the solution for nmap: i have to use my "real" (net) IP. so with 77.169.xxx.xxx it works just fine.

the problem remains: it's nmap which requires winpcap which are both no lightweight tools... i'd prefer something dead simple, something that doesn't install 2 unwanted programs on the client system.