you can attach any software you like if the game is supported even with a dynamic ip your ip only changes on reboot so as long as you know what it is you can give it out for people to connect/find you.
personally that software is junk and mainly only used by pirates because the likes of cod 4 still are populated by thousands of players who pay for there own servers but a legit player would never use gameranger because its another link in a chain thats to long anyway. my advice buy the game if you dont have it legit and play online and have fun. especially a game like cod 4 which i personally have put over 800 hours into. i payed £23 2 days after release so i have defiantly had my moneys worth out of it. i have ran clans and servers on it and theres still 5000+ players playing all the time. so theres no way you cant find a working server.