I had a similar problem with my wii,to get past the problem I inputted my wii mac number into the router, which sorted the problem.
To find the MAC address of your Xbox 360:
Go to the "System" area of the Xbox Dashboard;
Select "Network Settings";
Select "Edit Settings.";
Select "Additional Settings";
Select "Advanced Settings";
At the bottom of this screen you'll see a heading called "Wired MAC Address," which is your MAC address.