Start the Counter-Strike 1.6 game
Click on Favorites in the game menu
Click Add Server
Write your internal IP, for example, localhost:27015 or 127.0.0.1:27015 ( your internal IP can be different ( if you are building a game server on a different game port, then instead of the default port 27015, enter your port
)  example 127.0.0.1:27022
Click on Add this address to favorites
highlight the included address with the mouse button and click connect
Or you can use, a simpler way which is shown in the picture below:
Just in Counter-Strike 1.6 game console write: connect localhost:27015 and click Submit or press keyboard ENTER key.
your internal IP address, as well as the port, may be different.