If not, the problem is something else (sorry!) If 3.28.1 is the newest version, you need a fix. Look for Incompatible Kernel Extension Configuration Data.Under Software, click Installations to bring up that panel.Hold Option, then click the Apple logo at the top-left of your screen.If you’d like to force the issue, or don’t have Wi-Fi access, Apple has instructions for you, but here’s a quick summary. First, check if you’re running the affected software: If you’ve got a Wi-Fi connection, the problem should resolve itself as updates are installed. 9to5Mac reports that the broken update has been pulled, and an update for affected users is already available. It’s a pretty epic failure, to say the least. Version 3.28.1 of the “Incompatible Kernel Extension Configuration Data,” included in updates last week, blacklisted the BCM5701 driver necessary for most Macs to use their Ethernet port. Apple has a fix out, but if you don’t have access to Wi-Fi it’s going to take some effort.

Ghantous/FlickrAn update to OS X broke Ethernet on most Macs over the past week, frustrating users who prefer wired connections to Wi-Fi.

Mac ethernet problems