OS X 10.11 El Capitan includes the new System Integrity Protection (SIP) security layer, and Apple automatically pushes SIP updates transparently in the background using the same technology used to deliver XProtect updates to OS X 10.6-10.11. For Macs running OS X 10.9 or later, these technologies require having “Install system data files and security updates” checked in the App Store system preference pane.
Apple pushed down a SIP update in late February that included new list of blacklisted kernel extensions, and this had the unintentional side effect of disabling the Ethernet port on many Mac models. This means that some Macs running El Capitan that received this automatic SIP update can no longer connect to the internet via Ethernet.
Within 24 hours Apple pulled this SIP update, and pushed out a new one to correct the issue. Macs with non-functional Ethernet ports will need to connect to the internet via Wi-Fi to receive the update. If Wi-Fi connectivity isn’t possible, Apple has posted instructions for manually fixing this at https://support.apple.com/en-us/HT205956