So you have just purchased a new MacBook Pro with Retina display and you want to connect to your Ethernet network? The new 2012 MacBook Pro with Retina Display is now thinner and due to this reduction in girth the Ethernet port has now been replaced with another Thunderbolt port. To connect to any Ethernet based network you will need to purchase Apple’s new Thunderbolt to Gigabit Ethernet Adapter for $35 AUD.
The Apple Thunderbolt to Gigabit Ethernet Adapter lets you easily connect to a high-performance Gigabit Ethernet network. Small and compact, it connects to the Thunderbolt port on your Mac computer and provides an RJ-45 port that supports 10/100/1000BASE-T networks. – Apple
Source : Apple