Originally Posted by DrHow

I was experiencing the same symptoms on a Raspberry Pi 2 running Ubuntu Mate. This thread led me to the solution. Since my situation was slightly different, I am adding this comment which might help someone else googling for the same symptom.

Mine was a different problem in the area of ARP binding on my router, but I saw it because I was looking there. The RPi's MAC address was not being mentioned, but some other device was bound twice to 2 different IP addresses. I suspect one of those was the one that had been assigned the RPi. I deleted both fixed IP bindings and the RPi could see the WAN again.

Originally Posted by bartonski

even though you think you're connected via cat5 between Ubuntu and router, you're not (extremely unlikely that you'd do that by accident, but it's a logical possibility).