Please do NOT use ALXEthernet.kext. It is BROKEN with this motherboard (or just broken, period). You will randomly get crashes while transmitting ethernet packets (receiving works OK, so if you never upload, you may never trip over this bug, but it's there). I put up with this for ages, and it was horrible, before eventually figuring out that my ethernet was the problem. Then I got a USB Ethernet which fixed things for me at the expense of performance. I finally found the right driver while I was upgrading to ElCap.
There were apparently two versions of this motherboard, one with the Atheros 8151 and one with the Atheros 8161.
I have the Atheros 8151, and I'm using the
AtherosL1cEthernet.kext downloaded from
http://www.insanelymac.com/forum/files/file/374-atherosl1cethernetkext but you can compile it yourself if you like from code at
https://code.google.com/p/iats/downloads/list .
If you have the 8161, you'll need the AtherosE2200Ethernet.kext. It's safe to just put both in /L/E, or in /E/C/k/10.11, if you don't want to figure out which one you have.