I recently decided to play CoD4 mp, and at first it didnt work, i couldnt get any servers to appear, it says "Waiting for response from Master Server" and then gives the refresh time but no servers appear, so i added the port 28960 and it worked to start off with, i found servers and i played on them, but then i quit and restarted the game and refreshed the server list and i ended up having the same issue that i experienced the first time. At first i thought the master server might be down, but my friend was also playing at the time and could get servers to appear on his list.

I turned the firewall off that is on my computer and tried again but to no avail, so i was thinking it may be a firewall thats built into my router.

I've checked all my other settings in game like: source is internet, default filters, all types of games, but still nothing appears, I can't connect manually either.

I have the latest patch (1.7) and PB is up to date, so any help is much appreciated.

As you said, many routers have a built-in firewall these days, and you have to open the port 28960 in your router/firewall to be able to play.
Maybe you can find additional help here: PortForward.com

I'm currently having the same problems... Bought the game last spring, played single player, and simply never got to try Multiplayer. I've been too darn frustrated with this problem, so I just never tried it!

I went to a friend's house last weekend, and he let me play multiplayer on his system, and it was AWESOME! SO I decided to try figuring out WTH is going on with my system. Why can't I seem to get online at all to play? I just can't figure it out...

I've opened that port on my router firewall, I've completely disabled my PC firewall, and for some darn reason it's STILL not showing me any servers in the server list, so I still can't play COD4 Multiplayer.

Anybody have any CONCRETE SOLUTION to this problem???

ive been playing for ages now with no patches just plain old version 1.0 and all was good then i thought ill update it .....what a mistake i have not been able to play since all i get is NOTICE Server Disconnected on every single F****NG server the game loads map but then Bamm nothing just a lovely looking black screen i have tried i reinstalled back to basic and worked for 1 server but lagged like an old chinese movie and to honest i am getting majorley pissssssseeeeedddddd off i have updated PB and nothing why why why any help would be appreciated my firewalls ok, my comp is up to it , im up for it but game has had it i may just go back to BHD delta force. thankyou and sorry for the essay