I'm behind a firewall. Can I host or join MP games?

CM's official OFP FAQ says the following:

Problem: What ports need to be opened in a firewall to play (or to host) online? 

Solution: Direct X will need either TCP and UDP port 47624 or 6073, both incoming and outgoing as well as TCP and UDP ports 2300-2400, both incoming and outgoing. OFP itself requires TCP port 2234, both incoming and outgoing.

Norwegian OFP player Geir insists that UDP port 2234 is all that is required and not the equivalent TCP port, as Codemasters documented. So don't say I didn't tell you so!

In addition, for those of you using Gamespy Arcade to make your OFP MP connections, this support page will give you all the lowdown you need to know about Gamespy Arcade and firewalls.

Some ISPs place you behind a firewall without you being aware of it. If this is the case, you'll have to request them to disable their firewall for your account. If you do that, just make sure you protect yourself minimally with a product such as Zone Alarm.

If you're still experiencing problems, maybe OFP player FFX_haXor_FFX's experiences can help you further:

"For reasons beyond our control, a good majority of users cannot or will not open up a range of ports, such as 2300 to 2400, as this is too much of a security risk to.

But we can "compromise". OFP uses the next available high ports: 2235, 2236, 2237, etc. Opening 2235 seems to have allowed a good majority of the packets through.

However, I did notice a lot of rejected UDP packets from port 2235 on my external interface but not from 2236, 2237, etc. At first, I didn't understand why but OFP seems to use UDP port 2235 to transfer response information to and from the server. I opened TCP port 2235 for good measure but it doesn't seem to need it .

I am still having some time-out issues on my end but I sense some packet loss elsewhere as well or maybe its just my lousy ISP connection.

CURRENT ISSUE: I sit in the lobby for a few minutes (2 to 5) and I get a "session lost" error message. If I click past the Lobby to "roles selection", I am ok - the game plays through fine, with no lag at all. 

This is where words like "netcode" and "beta" come in. If the game truly needs 100 ports, we are all in trouble. So I shall wait for the fully updated netcode and hope the issue of massive port requirements is corrected in OFP's next major game code revision."

(Source: this OFP Network forum thread).