OK, this is just F'ing weird. I have a server set up running Ubuntu 12.04. It's hooked to an Asus RT-n66u via ethernet cable. Also wired to the router is my 27" iMac.
Twice in the last week (and once many moons ago) I've come in to find no internet connection on my Mac. The first time I went through every networking diagnostic thread I could find with zero success. No ethernet connection, but internet works fine when I fire up the wifi, and every other device that connects via wifi is fine. Finally, for some reason, I plugged a spare keyboard and monitor into the server only to find it non responsive.
OK. Hard shut down and ethernet is instantly working again on my Mac.
It just happened again and I killed the server. Voila, ethernet fixed.
So. WTH is going on and how is one device breaking the ethernet connection to another device but leaves wifi AOK?