Well, opening ports can be a different process, depending on what brand your router is. I'm assuming you're using Linksys, which is what this small guide will be based on:
1. Open internet explorer.
2. In the address bar, type "192.168.1.1" without the quotes, and press enter.
3. Now a prompt for a username and password should appear. This is normal. If you're using an older Linksys router, leave the username blank and the default password is "admin" without the quotes. If you're using a newer router, the username is also "admin".
4. Now you should be at the "General" settings page, assuming that the above step worked for you. You should see a tab called "Advanced" in the top right corner of the screen. Click it.
5. Now you have a whole new set of tabs occupying the top row of the screen. Find the tab called "Forwarding" and click it.
6. Ta-da! Here is the port-forwarding settings page. The ports that Half-Life & mods use are 27000-27030 with both UDP and TCP enabled.
7. Once finished entering the ports, click the "Apply" or "Apply Settings" button at the bottom of the screen. You should now be able to successfully host an internet ESF server.
Hope this helped!