What MAC address should I use for my Thunderbolt Ethernet on a MBPR? I use MAC addresses to assign static IPs on my network and have recently added a new MacBook Pro Retina with Thunderbolt adapters in stead of Ethernet ports. On the box one MAC address is given for the "Ethernet" but when I plug in to the Thunderbolt ports using the Ethernet-Thunderbolt adapter, I see a different MAC address for this connection.

Which MAC address does the DCHP server see when assigning IP addresses?



I should add that the Ethernet-Thunderbolt adapter doesn't seem to work at all for me. It either incorrectly reports that no cable is connected, or self-assigns an IP address.
When an interface self-assigns an IP it generally means the interface isn't getting any response from your router or DHCP server. This is a feature of OS X to let the machines build an adhoc network when they cannot determine the actual network they're on -- you can put a bunch of Macs on a dumb hub and almost get them to self-organize and function with this feature.

Your router and DHCP server are going to see the MAC address listed for the device in the Network Preferences pane. Go to System Preferences -&gt; Network and click on your Thunderbolt interface. Click on the Advanced... button and then on the Hardware tab:



That's the MAC address that your interface is broadcasting on the network when it's trying to acquire an IP on your network from your DHCP server.