I see a lack of understanding here.
When your computer connects to the internet, what it actually does is to connect to your router, which then acts as a gateway to the larger internet.
Your PC's IP is not visible to the Internet (and by extension, the fs2netd server), your router's IP is. This is what's called an external IP.
You can use a site like http://www.whatsmyip.org/
to find out what your external IP is.
If two PCs are behind the same router, they will share an external IP, and thus look to the internet like one single machine.
The Launcher's "Force IP" will not work over the Internet, as it just forces your local IP to whatever you set there. Which is a bad idea, possibly won't work, and should only be used if you really, really know what you are doing.