Because the developers of Ethernet had the vision to use a 48-bit address space, there are a potential 248 (or 281 trillion) MAC addresses. Ethernet MAC addresses are typically given as a string of 12 hexadecimal digits. The first six digits identify the manufacturer of the card (comprising the Organizational Unique Identifier, or OUI), and the last six digits are assigned by the manufacturer (comprising the Burned-In Address, or BIA). The IEEE assigns the 24-bit OUI prefixes to organizations by allocating blocks of 224 (that is, about 16 million) MAC addresses at one time. In short, MAC addresses can be used for the authentication of computers.
MAC addresses of modern network cards can be changed to arbitrary values. Thus, mechanisms based solely on MAC authentication are susceptible to spoofing attacks.