I explained this in another topic, however, I just re-searched my post and C&P'ed what I said there.
Ok, well first off you can try using your DMZ.
Firstly, you will need the IP of your xbox 360. To do this, go to the purple blade (last blade over to the right).
Then go to network settings > edit settings (don't worry you won't edit anything in here) > note down IP.
Now go into your router's admin panel, and look for the tab "DMZ", in there you can add a few IPs to the DMZ. Add in your xbox 360 IP address in there, voila, should be open. (works for me 95% of the time)
Note: oyu can remove any PC Ips in there if you wish/need space.
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If that doesn't work you will want to port forward to open them.
The prescribed solution is to "open" the following ports on your router:
* UDP 88
* UDP 3074
* TCP 3074
By "open" I mean forward traffic on those ports to your Xbox 360 console. First, you need to find the local IP of your Xbox 360 console. You can get this from the network settings tab in the dashboard. It's probably 192.168.x.x. Now, login to your router's admin panel. If you've never done this, refer to your router's user manual. It's not difficult at all. If you're absolutely too lazy or unable to find the manual, try the following:
1. Type 192.168.1.1 into your browser's address bar. If that doesn't bring up your router's admin panel try 192.168.1.100 or 192.168.2.1
2. When you get the login screen for your router try the following common default user name/ password combinations UN: admin PW: admin UN: admin PW: blank (I mean literally, leave it blank) PW only: blank
3. You're looking for a setting that says port forwarding, virtual servers, UPnP forwarding, or something similar. Bottom line is find a screen that looks like the one below, and enter the above settings in addition to your console's IP address. As you can see, my console's IP is 192.168.2.3
I hope I helped.