The most common cause of this issue is that security software (such as a software firewall) or hardware (such as a router) is blocking connections from coming into your network and making a connection to the host machine. You will need to open/forward those ports to the host machine.
The following is a list of ports used by Killing Floor. Make sure they are open in your router/firewall. (And directed at the host machine's local network IP)
7707 UDP/IP (Game Port)
7708 UDP/IP (Query Port)
7717 UDP/IP (GameSpy Query Port)
28852 TCP/IP and UDP (Allows your Server to Connect to the Master Server Browser)
8075 TCP/IP (Port set via ListenPort that your WebAdmin will run on)
20560 UDP/IP (Steam Port)
Another setting you might have to deal with when hosting a listen server in your killingfloor.ini is:
ServerBehindNAT=False Set it to True
Here is a internet tool you can use to test if your ports are open -
GRC Shields Up
