So you know that a mac address of 0xDEAD BEEF FEED (from above) is kinda a joke right ? And you know that having the same mac on two devices breaks your network.
A media access control address (MAC address) of a device is a unique identifier assigned to a network interface controller (NIC) for communications at the data link layer of a network segment.
MAC addresses are most often assigned by the manufacturer of a NIC and are stored in its hardware, such as the card's read-only memory or some other firmware mechanism.
The Arduino ethenet board this is replacing has the mac assigned and printed on a paper tag on the back.