Are MAC addresses really unique?

Are media access control addresses really unique? Or are there any (maybe cloned?) network interface cards that have the same MAC address as another NIC? What is the probability of having two identical MAC addresses within one network?

The IEEE manages MAC addresses. The hardware identification addresses that the IEEE distributes are unique. That makes the probability of matching MAC addresses zero.

On the other hand, some hardware MAC addresses are programmable, which makes them spoofable. This means that it is possible for two machines in the same network to have the same MAC address.

To actually calculate the probability that two or more computers in the same network share the same MAC address, look up ‘selection with and without replacement’ in an introductory Combinatorics or Probability and Statistics textbook and follow the procedures outlined in the book.

You can find network standards information available online at http://standards.ieee.org. In addition to the FAQ files there, the IEEE recently has made the collection of 802 Ethernet networking standards documents available free of charge in PDF format through the Get IEEE 802 program.