First of all let me explain you what is NAT. Network Address Translation (NAT) is the process where a network device, usually a firewall, assigns a public address to a computer (or group of computers) inside a private network. The main use of NAT (Generally Speaking) is to limit the number of public IP addresses. Althought there are many types of NAT, the well-known on this game are the the Strict, Moderate and Open NAT types.

If you play Call of Duty: Modern Warfare 3 (or any other P2P based multiplayer game), you should consider setting up a port forward for it to better your online gaming experience.

There are several advantages to setting up a port forward for Call of Duty: Modern Warfare 3, for example:
- Not getting dropped from games
- Better quality voice chat (in game)
- Less lag
- Improved connection
- Easier to play with friends / join lobbies
- Overall enhanced online experience
...

Note:
This will not speed up your internet speed!
So if you have a very bad internet connection open ports will not fix this "speed problem".

As the title says, I will explain you in a few words the main diffrences of Strict, Moderate and Open Nat.

•Strict NAT: With this kind of NAT you will be able to connect only with players who have their NAT type opened. Also searching for games will take more time. At 90% you will be kicked when a host migration comes up and also don't forget that lags will be possible.

•Moderate NAT: With this kind of NAT, you can connect up with people who have either Moderate or Open NAT. Searching for games with this kind of NAT will take some time, but not as much as Strict. Lags wil be less than Strict to.

•Open NAT: With this kind of NAT, you will be able to connect to any of the three types. Searching for games will require less time and you minimize the chance of getting lags or kicks during a host migration.

On this guide I will show you, 2 methods in order to get an Open NAT type. The first method is easier,faster (takes just 2-3mins) but temporary. It's recommended for new players of Modern Warfare and generally speaking with people who don't have enough experience. The problem is that it's temporary. Everytime you restart your router or modem everything is reset. But as I stated above, even with that it will take you just 2-3 min. The second method requires a bit more experience but it's a peramanent solution to your problem. You have to edit your router's cfg file.

TIP:At 80% you can skip the steps below except the UPNP and go directly to method A or B.

Before you choose your method, you have to forward the right ports. Don't worry, I will show you how. Let's start:
• Go start and type cmd then press enter.
• A window will pop up. Type on the command prompt (cmd) "ipconfig".
• Find the default gate address and write it down. In most cases it's 192.168.1.254
• Open your web browser and put the address which you have written down in the addrees tab.
• A window will pop up requesting your router's login info. Put your info there and your router's page will load up.
• Go to the port forwarding section of your router ( In Thomson routers it's the game and application sharing founded on the toolbox). Make sure to have enable your UPNP. Then you must forward the right ports (If you don't know how to port forward, you should download your router's manual or just visit the following site: http://portforward.com/ This site has also guides on how to port forward in your routers).
• The ports for TCP are 3074, 27014-27050 and for UDP are 3074 27000-27030.
• Apply your settings then reboot your router.

After you have rebooted your router / modem, you will follow theese instructions:
• Double click your icon "My computer". The window will pop up. At your left hand you will see an option called network. Click it.
• The page will load.
• Right click on Network Infrastructure and click properties.
• A window will pop up again. After that click Settings. You have to wait for a couple of seconds.
• A new window will appear. Click add at the bottom of the window.
• For one more time a new window will appear. Then do the following: in the first tab (name of service) put IW, in the second tab put your IPV4 address (To find it, go on cmd then type ipconfig.),
in the third tab put 28950 and don't forget to put UDP, and in the last tab you put again 28950. Then click ok.
• One more time click add. In the first tab you put MW3 OPEN NAT(or whatever you want I just prefer MW3 OPEN NAT), on the second tab you put your IPV4 address, in the third tab 3074 and don't forget to put UDP and in the last tab you put again 3074.
• You click ok, then again ok, then again ok.
• Launch Modern Warfare 3. Your NAT type should be opened.

Again I remind you that you have to do the procedure above every time you reboot your router or restarting every active connnections.

This is the method in which you will permamently fix the problem.
• Log in, on your router's page using one of the browsers.
• Go to your router's configuration page.
• Somewhere there must be an option called "Save or Restore Configuration". Click on it.
• A new page will load up. Use the option "back up configuration now.."
• Wait a couple of seconds and a window will pop up. Then click, save file, then ok.
• Make 2 copys of this file so if something's go wrong we will have a backup.
• Open the file.
• Hit down Ctrl + F and write [ connection.ini ]
• When you will find that, look for the last bind.
• Under the last bind paste this: "bind application=CONE(UDP) port=3074-3075" then under the last one paste this "bind application=CONE(UDP) port=3478-3479" then under the last one for one more time paste this "bind application=CONE(UDP) port=3658"
• After that save the file (Probably the file is opened with notebook.)
• Then go back to your router's page at the configuration tab where you have been before.
• Use the Option to browse your new configuration file.
• After that click "Restore Configuration Now". Be patient and wait.
• Close your router's page and reboot your router. When you launch Modern Warfarre 3 NAT should be Open.

•Method A works for Windows Vista (Service Pack 2), Windows 7, Windows 8 and 8.1, and Windows 10
•Method B works with the ones written above plus with the XP OS.
•If you finally didn't managed to open your NAT type, don't get angry or sad. Relax a bit and carefully follow step by step this guide. If you still have problems, feel free to add me and I will fix it for you.

This is my first guide so far. I tried to explain you with the simplest way I could, everything about NAT. Don't be angry or rude with me if I am wrong at some points. I will add soon videos and images for easier access. I would be greatfull to leave your comments so I would make the guide even better. Also if this guide worked for you, please Rate it and Favourite it. Thanks a lot!

UDP 27000 to 27015 inclusive (Game client traffic)
UDP 27015 to 27030 inclusive (Typically Matchmaking and HLTV)
UDP 27031 and 27036 (incoming, for In-Home Streaming)
TCP 27036 and 27037 (incoming, for In-Home Streaming)
UDP 4380
https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711


https://support.activision.com/articles/en_US/FAQ/Port-Forward-and-NAT-FAQ
https://support.activision.com/articles/en_US/FAQ/Ports-Used-for-Call-of-Duty-Games/?l=en_US&c=Game_Title%3ACall_of_Duty_Modern_Warfare_3&fs=Search&pn=1

Xbox 360
TCP: 53, 3074
UDP: 53, 88, 3074

Playstation 3
TCP: 5223, 3074
UDP: 3074, 3478, 3479

PC
TCP: 3074, 27014-27050
UDP: 3074, 27014-27030


MW3 Optimizations Summary:
http://forums.steampowered.com/forums/showthread.php?t=2225829

How to reduce the "lag compensation" MW3 adds to your client!
The MW3 engine tries to balance the matches by adding lag to people with high-performance computers and connections - which means: if you have high FPS and GPU usage and low ping or host, the game will see this as an advantage and add latency to your client. This means that the more GPU usage you have - the more time advantage your enemies will have over you!
To fix this....
http://forums.steampowered.com/forums/showthread.php?t=2253196

Can a open NAT cause problems?

Well, if a hacker uses a stack overflow bug in MW3 that is exploitable over a network and has a couple 100 bytes spare in its buffer while online playing, then your computer is pwned. Fortunately they patched this bug couple years ago. Plus the lousy P2P network in MW3 (and MW2 or AW) is most of the time used to capacity and offers no extra bytes spare.

Now going back to your original question, you might have read in some forums that you should avoid open ports to avoid any accident with an exploitable bug in a program. But you now have a second reason: the remote shell that a hacker would then use is another open port. If you have a firewall that is blocking everything except that you have specifically allowed (games, steam, skype etc.), you would also block that remote shell.

• Open port: When anyone asks, the computer responds that there is a service listening on this port. This means anything coming to this port will get processed by a program (a service) running on that computer.
  
• Closed port: When anyone asks, the computer responds that there are no services listening on that port. The akser will know there is a computer responding at the address.
  
• Stealth port: When anyone asks, they get no reply. The point is to hide if there is a computer at the address at all. Well, for real good coders even this might not be very effective.

If you have an open port, you are safe provided the program processing the incoming stuff has no available exploits. But exploits are found all the time, and it's good to know that there are a lot of port scans travelling around the net, looking for targets.

Closed ports still respond to the asker, so possible attacker knows to proceed checking other ports. Then again, this is how the internet is specified to work. While stealth ports attempt to not give the potential attacker any information, in theory they break the specification.

From a security point of view, any open port is a huge gaping hole, since code is being used to process foreign data. What a firewall (or a NAT router) does is make sure no incoming traffic gets to your computer, even if the system has some open ports. This way, they effectively close all ports.

1. A port isn't open if something isn't listening for a connection on it.
2. (Most/Newer) routers firewall can be set to allow only specific game applications through
3. Follow the basic security rules: keep drivers/software/firmware (router!) up to date.
4. Default settings should be changed to best settings!


True Hacking (not cheat/tools-downloading) is not so easy as many people think.
TBH a real “hacker” would still be able to do other nasty things to your computer, so don't be naïve. But why would such a skilled “Pro-Hacker” spent his time to harm your PC?