I can't join my friends xbox live party or games. We both have open nat. I can join other parties, just not those hosted by him.

You should make sure your NAT is open ! This allows you to communicate with fellow gamers ! Go to your router settings and find DMZ setting. Once found find your xbox IP address and enter it into the DMZ ! This should work if not post back!

You are trying to open the NAT on the router for X-box.
For this you will need to reduce the MTU settings on the router to 1365.
You will also need to trigger some ports on the router.
For this you will need to open the router set up page.
Router set up page is the page where you can change the settings of the router

Click Here to know more about router and network

if you try power cycling your modem and router it usually works, otherwise youd have to have someone else host and both of you join, then have the host leave. just microsofts way of keeping people apart lol

your friends nat are not.

well one thing that comes to mind is his NAT type mite have changed that can cause you to not be able to join game or chat and or xbox live partys so you can ask him if he can check his NAT type.

good luck :)

Open or forward ports on the router for your Xbox. Get the ports from Xbox live then configure the router to allow those ports to pass.

The case is the way the network is setup are you connecting him via a router? Make sure your NAT is open on the router as well.

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That happened to me once. You should check the wireless device or your ethernet cable( whichever one your using ). If your ethernet cable isnt plugged in on the way it will only let you log into xbox live and that may be it. Lastly try resetting your router and make sure all the wires are plugged in good.
Also Check your NAT settings. If your trying to connect to a friend, You or your friend most probably do not both have an OPEN connection. To find out if you do or dont, go to your xbox's network settings and run a test, if it comes up with a "More Information" button click it, and it will tell you what type of NAT, your xbox is set too. However if it does not come with a More Info button, then your xbox is OPEN.
Make sure both you and your friend do these tests.
If one of you is not OPEN, then you will need to either open some ports up or turn on DHCP and UPnP. But I wont go into detail until you've done the test.
Hope this helps.

your nat type is strict you can change this by opening up your internet explorer go to tools then internet options and make the needed changes

tell your friend to set his party to anyone and not just friends or invite only. its in a settings tab within his party options