A MAC address is made up of 6 bytes written as 12 hex digits. For example: 00-80-C8-E3-4C-BD. Of this address, the first 3 bytes (00-80-C8) identifies the card's manufacturer. The IEEE is the body that assigns manufacturers their IDs, called Organizationally Unique Identifiers. So, this leaves 3 bytes which can be assigned by the manufacturer to be unique to that manufacturer. So, each manufacturer can have 16,777,215 different unique ID's for their cards (00-00-00 to ff-ff-ff). There are some unique MAC addresses though such as all broadcast ff-ff-ff-ff-ff-ff or different multicast MAC's 01-00-5e-00-00-00 through 01-00-5e-7f-ff-ff. So, really with the number of manufacturers and unique addresses, does it seem like running out of MAC addresses is realistic?