Are you having trouble finding games on IWNet’s matchmaking service? Are you having trouble joining games off your friend list? Are your friends having trouble joining your games? Are you getting disconnected randomly? Are you being dropped during host migration? Are you losing confidence, self-esteem, and the will to live?

Your problem is most likely your NAT configuration. Or, in the last case, maybe just bad breath or grotty shoes or a totally repellent personality or something.

To look really smart and everything, I’ll start by defining NAT with the power of my huge brain (but probably actually just a sentence I cribbed from Wikipedia):

In computer networking, network address translation (NAT) is the process of modifying network address information in datagram packet headers while in transit across a traffic routing device for the purpose of remapping a given address space into another.

In human speech, this is basically translated as:

In computer networking, network address translation (NAT) is the process of stuff.

The upshot is that remote connection requests being shoved through your router are breaking because your network clients have their own IP addresses, which are different to your gateway IP. If that doesn’t make sense, just move on. It doesn’t really matter what it is, anyway. The only really important bit for you is that NAT comes in three types – strict, moderate, and open. For the purposes of gaming online with any sort of end-to-end connectivity, you want your NAT to be open. It’s also worth mentioning that even a single player in a game with a strict or moderate NAT can potentially cause connection disruptions for everyone else. Don’t be that player.

There are three broad categories of connection type I’m going to distinguish right off – you’re either connecting to the internet through a modem hooked up directly to your PC, through an Internet Connection Sharing (ICS) setup, or some sort of router/modem combo.

In the first case, your problem isn’t actually a NAT issue at all, since you’re connected to the internet about as directly as it’s possible to be, and there’s no NAT involved whatsoever. If you’re using this sort of setup and being told you’ve got a strict or moderate NAT, it’s probably because your firewall behaviour looks that way to IWNet and/or Steam (i.e. it’s refusing connection requests). So it’s nothing to do with NAT, and everything to do with your firewall blocking access on IWNet’s and/or Steam’s service ports. Open those ports (listed below), or shut down your firewall.

In the second case, the overall context is a little more complicated, but the solution is the more or less the same as above. “More or less”, that is, because the option of shutting down the firewall is (much) more likely to be the only solution here.

The last case is going to be the most common scenario, and it’s very easy to fix indeed.

1.    Start off by assigning your gaming PC a static IP address, if it doesn’t already have one. I’m going to repeat this, because if you don’t do it, this isn’t going to work. So start off by assigning your gaming PC a static IP address, if it doesn’t already have one. This procedure varies somewhat between operating systems, but is likely to include visiting your network browser, identifying your LAN connection, and changing its relevant property (the IP address). Most basic home networks use addresses in the 192.168.x.x range, so unless you’re some sort of monstrous aberration, you’ll be going with this too. If you have no idea what range your network is using, then before changing anything, jump to your command prompt and type ipconfig -all. Your current (automatically-assigned) IP address will be listed there. Just copy that.

2.    Open your internet browser, and log in to your router (usually 192.168.1.1 or 10.1.10.1). Obviously model settings are going to vary, but assuming you’re using a router worth using, there’s going to be a section for port opening policies (it may be listed under its security settings). It should be a table of some sort, with headings like application, start, end, protocol, IP address, and possibly a checkbox.

3.    For application, just use MW2.1, MW2.2, etc. The ports and protocols you’re going to set are:

UDP 1500
UDP 3005
UDP 3101
UDP 28960

And just for good measure, let’s make sure Steam doesn’t stuff anything up, and add its service ports too:

UDP 27000 to 27015 inclusive
UDP 27015 to 27030 inclusive
TCP 27014 to 27050 inclusive

4.    For the IP address, set the static IP you’ve already assigned to your gaming PC.

5.    Save your settings.

6.    Gloat to your friends about your shiny new open NAT.

In the exceedingly unlikely event that problems persist, roll on over to www.portforward.com, and locate your router model there for specific instructions.

Discuss Modern Warfare 2 connection settings in the forums