Here's an odd question: How do you release a DHCP address in Ubuntu 12.04?

I had some performance issues with my Internet connection this morning, so I hooked my Ubuntu laptop for some quick diagnostics. Unfortunately, my ISP only assigns one IP address at a time via DHCP. To get my router back online, I'd either have to manually release the DHCP lease from my Ubuntu laptop, or wait several hours for the lease to expire.

I figured it would be as simple as going to Network Manager and disconnecting my Wired Connection before I physically unplug the cable, but while that does bring down the interface, it doesn't release the DHCP lease. I also tried running sudo dhclient -r, but that doesn't do anything.

So.... yeah. Now what?