The port you'll want to forward is probably 27015, although this is configurable on the command line to HLDS.exe, so it might vary. You'll want to forward it to the IP address of the local machine that is running the server.
For instance, if your local machine's IP is 192.168.0.50, and you've got HLDS set up on 27015, you'd need to set the router up to forward both TCP and UDP on 27015 to IP address 192.168.0.50
It allows a user to access a specific service like webpage, gaming, security camera setup, VoIP, and downloading files by identifying it's port number.
Enabling port forwarding: Advanced Port forwarding
1. Click ADD rule button under Port forwarding rules section & enter following details.
2.Name: Enter the service or game you would like to host from your computer
3.Protocol: Select TCP/UDP
4.External Port: Enter the port number
5.Internal IP address: Enter the IP address of your local computer that will provide this service
6.Internal Port: Enter a random port number for hiding the original one and click ‘Install
User can do it in 3 ways:-
1.UPnP (Universal Plug & Play): Advanced Additional Plug and Play Enable
It permits networked devices, such as PC, printers, Internet gateways, Wi-Fi access points and mobile devices to seamlessly discover each other's presence on the network for data sharing
2.Port Forwarding: Advanced Port Forwarding
Enabling Range forwarding, you can open a range of ports but in case of port forwarding you are opening only one port.
3.DMZ (De-Militarized Zone): Advanced Port forwarding DMZ Turn On .
DMZ will forward all the incoming traffic to a single specified IP address on you local network. i.e you are opening all ports on that specific IP. It's not secureda as It is totally exposed to the internet
Steps to enble DMZ:
1.Turn On the DMZ
2.Type the IP address of the device to be relayed to WAN
3.Click the Apply button.
4.To stop relaying a computer/device to WAN, simply Turn Off the DMZ.