QUOTE(TheMuffinMan @ Dec 1 2009, 03:32 AM)
DNS is handled by your ISP.
Assuming you have the correct cabling (patch cable if from router to Xbox) or are wireless then the DNS should be received from the ISP.
Do an "ipconfig /all" on your computer and see what the DNS settings are. Most likely it's the router's IP address if you have one configured. Your Xbox should automatically detect this as well.
What is your exact network setup?
I've done the "ipconfig /all" and gotten the 2 DNS numbers, put them in manually. Nothing will be found automatically.
I have a router that I have never had a problem with before, even if I switched from the router to just directly connecting the modem to the 360 it wasn't an issue. Even now I have tried both and neither are working. Think it could be my NIC card gone bad or something? I recently switched ISP's as well and when I call either Xbox support or my ISP support they always tell me to contact the other one.