After recently replacing my main router on my home network with a Cisco e4200 and updating it with DD-WRT, my xBox refused to get an IP address from the DHCP server. I could see the IP lease on the router and even after deleting the lease it would generate another lease. But the xBox itself would not use the IP address.
I noticed the default lease lifetime was set to 0 and thought I’d try changing it to something else for the heck of it. I set it to 1440 (which is 24 hours) and voila it worked. All other devices were functioning ok at 0, not sure why the Xbox didn’t like it. Oh well.