Unfortunately, this fix does not work when a MAC address is not even showing even though the same previously working ethernet cable plugged in and no changes were ever made to a system running the latest OS X El Capitan (10.11.3) as of the date of this post. No changes also include to the network it is connected to. That is, no router, switch, port, or hub changes. No patches or updates were applied. Tried running First Aid after holding CMD + R on start-up from a full shut down with the internet recovery process was commenced in an attempt to have the ethernet card driver re-installed. No success. The following screenshot images showed identical prior to and after First Aid was attempted with no changes