This is a thread detailing on how to connect to Xbox Live with an Open NAT with verizon FiOS. At least with the Actiontec Router anyways.
Verizon FiOS with Actiontec Router connecting to Xbox Live Tutorial
Step 1: Go to your Xbox's Settings Blade and go to Network Settings. From there, go to Edit Settings. You probably will see a IP Address that looks looks like this on the top of the screen: 169.xxx.xx.xxx. Press A to edit the IP Settings Panel. Scroll down to Manual and you will see four options. One of then is Done so you don't need to worry about that. First, click on IP Address. Set it to 192.168.1.109. Press Start to finish and then go to Subnet Mask. Enter the numbers 255.255.255.0. Click Start to finish. Then, go to the third option (Gateway) and enter the IP address of your router. This is normally 192.168.1.1 for most FiOS users. Finish by clicking the Start button again Then, scroll down to the Done button and press A. A blade with come up with the options to Test Media, Test Xbox LIVE, or Cancel. Click Test Media. The test should fail the IP Address test. Scroll down to Edit Settings and click. Go to the second pane now, and click. Scroll down to Manual and then click Primary DNS Server. Enter the same info you put in earlier for the Gateway setting and press Start to finish. Scroll down to Done and click. Once again, a blade will come up and once again, click Test Media. Once again it will fail the IP Address test. You are now done step 1.
Step 2: Get on a laptop (or desktop with Wi-Fi) and go to 192.168.1.1. There is no period behind the last 1. The standard user name is "admin" and the standard passwords are "password" and "password1". Once you're into the router, you'll see a row of icons at the top. Click Advanced and say Yes when you are told the dangers. On the left side of the Advanced tab is a red toolbox. Under it, there is a button that says Restore Defaults. WARNING: YOUR XBOX MUST BE ON DURING THIS NEXT PART. Click it and click Apply when prompted. This is where things get tricky. Go find the physical router in your house/office and find a little white sticker on it. Record the SSID and WEP Key. Return to your laptop. The SSID you just recorded should be listed as a wireless network. Click on it and enter the WEP key when prompted. You are now back online. Go back to 192.168.1.1 and it will prompt you to make a new user name and password. Just make one with your last name and first name as your password. This just makes things easier. If you did it right, then at the bottom of the My Network tab should be a device with a IP address as a host name. If it's the IP address you set for your Xbox earlier, then your good. To make things even easier, go to the Wireless Settings tab and click on Basic Settings. Change your SSID to whatever you want and you can also disable WEP from here if you want. If you choose not to disable, then remember the one you wrote down earlier. Now your done step 2.
Step 3: Get back on your Xbox and go to the Edit Settings page. Go to the bottom pane and click. It should pop up with a list of networks and one of them should be your network with the new SSID. If it warns you that you are connecting to an unsecured network, connect anyways as it just means there is no WEP key. A blade will appear prompting you to test Live or Media, or cancel the operation. Click Test Media again. This time, it should pass the IP Address test but fail PC Connection.Scroll down to the Edit Settings option. Then press B. Now, go to the Test Xbox LIVE Connection option. It should pass all of the tests and give you a NAT Moderate message. Time to fix this.
Step 4: Go back to your laptop and go back to 192.168.1.1. Once you log in, go to the Firewall Settings tab and say yes to all the warnings. On the left should be a bunch of options with one of them being DMZ Hosting. Click. First, enter the IP Address that you punched into the Xbox earlier. Then check the box next to it and hit Apply. Wait for it to bring you back to the warning screen and click yes, just to be on the safe side. Then go back to your Xbox and do the Xbox LIVE Connection test again. This time it should pass all the tests and it should give you a NAT Open rating. Bam! You're done.
Note: When you sign into Live you may get a message saying that it can't connect to Windows Messenger. Disregard this message as this would involve some more technical work. If I get enough demand, I'll post a tutorial on doing that too.
Let me know if you are having problems with MTU or DNS.
I've seen enough threads on many forums of people saying they can't connect. Well, neither could I for around 3 months. This works with my 360 wireless adapter and the Actiontec router.
Credits: Ian Pederson