Killing Floor problem?

Not needed if you don't host. I have a look what I can dig out.

I've played KF with a router, didn't have such a problem. Ports are used for others to be able to connect to you, so if you host the server yourself - you need to open ports, otherwise I doubt it.

Question: Which router do you have? It seems to have problems with some routers.

Although it might not be the reason for your problem,I just post this here.

To find out if the router is the problem try pluggin yourself into the modem(no router). If that does not work/or is not possible) I suggest you also try making a post in the tripwire forums,they are better supported for troubleshooting stuff.

Edit: Yeah,sorry,just reread and had already forgotton you tried without the router already. Check if its one of the routers mentioned and try the workarounds,if that does not help just try to get some help on their forums (or from support directly,Tripwire may be small,but they try to do their best)