First, you should know the IP address of the PC that's hosting the server (this will be one of the non-routable IP's served up by the NetGear). If you only have one PC, it'll be 192.168.0.2 by default. And, I'm assuming you're running your Half-Life server at port 27015 like normal.
Second, make sure your NetGear's on the latest firmware (v3.22). You can get it from www.netgear.com. Also, be sure to read the documentation that comes with the firmware upgrade; the manual has been updated for this feature, and it's described in Chapter 6.
Once upgraded, connect to the router by doing the following: Start - Run, then enter "telnet 192.168.0.1" . Enter the password (the default password from the factory is 1234). At the main menu, choose menu 15. You should see something like this:
Cursor down to line 2, then enter 27015 for the port and 192.168.0.2 for the address. Then keep hitting enter all the way down the page until your changes are saved to ROM and you're back at the main menu.
Here's what mine looks like now:
What this means is that all traffic your router receives to port 27015 will be automatically forwarded to port 27015 at IP address 192.168.0.2. This is very cool, and it'll work for other things too, like if you want to run a Web server (port 80) or make FTP available (port 21).
However, there are some side effects. For example, you can't run more than one Half-Life server at port 27015. In the above example where we have the first machine running Half-Life at port 27015 on 192.168.0.2, if you added a second machine, it would be IP 192.168.0.3, and you'd want to run Half-Life at, say, port 27020 or something like that. Then just add the appropriate line into menu 15, e.g.
More details can be found in the documentation, but this should give you a basic start.
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