Alright, this is actually a pretty simple problem, but only once you know which configuration files to look at.
I recently replaced the machine that powers this very web site with a better one. This was my first migration since switching to Ubuntu Server last winter. I essentially took the hard drive out of the old machine and plopped it in the new one, booted it up, and hoped for the best. Since Linux is not as picky as Windows about being moved to a new set of hardware, I figured it would work out fine.
Sure enough, Ubuntu booted right up on the new machine without so much as a complaint. However, network connectivity was gone. The old machine was using a PCI Ethernet card, while the new machine had an on-board controller that I hoped to use. Anyway, I took the PCI card from the old machine and put it in the new machine, and then the network connectivity was back.
Why wouldn’t Ubuntu just start using the on-board controller, though?