Media Access Control better known as MAC is a unique hardware number that identifies each hardware device. Other names for these are physical addresses, hardware addresses, and Ethernet devices. There are three numbering systems that the Institute of Electrical and Electronics Engineers, Incorporated (IEEE) manages MAC-48, EUI-48, and EUI-64. (The "EUI" stands for Extended Unique Identifier.) Computrols uses MAC-48. All three numbering systems use the same format and differ only in the length of the identifier. Addresses can either be "universally administered addresses" or "locally administered addresses."

A "universally administered address" is uniquely assigned to a device by its manufacturer; these are sometimes called "burned-in addresses." The first three octets (in transmission order) identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI). An Organizationally Unique Identifier (OUI) is a 24-bit number that is purchased from the IEEE Registration Authority. This identifier uniquely identifies a vendor, manufacturer, or other organization (referred to by the IEEE as the ‘assignee’) globally or worldwide and effectively reserves a block of each possible type of derivative identifier (such as MAC addresses and others) for the exclusive use of the assignee. Computrols owns the OUI, 00:09:14. The OUI is subsequently used by the vendor, manufacturer, or organization (the assignee) to create particular instances of these identifiers for various purposes, such as the identification of a particular piece of equipment or the identification of a network protocol, and for use in various computer hardware products. The IEEE expects the MAC-48 space to be exhausted no sooner than the year 2100.

A "locally administered address" is assigned to a device by a network administrator, overriding the burned-in address. Locally administered addresses do not contain OUIs.

MAC-48 and EUI-48 addresses are usually shown in hexadecimal format, with each octet separated by a dash or colon. An example of a MAC-48 address would be "00-09-14-4C-7F-1D". The last three octets represent the serial number assigned to the adapter by the manufacturer.

The following technologies use the MAC-48 identifier format:

• Ethernet
• 802.11 wireless networks
• Bluetooth
• Fiber Channel and Serial Attached SCSI (as part of a World Wide Name)

The distinction between EUI-48 and MAC-48 identifiers is purely semantic: MAC-48 is used for network hardware; EUI-48 is used to identify other devices and software. (Thus, by definition, an EUI-48 is not in fact a "MAC address", although it is syntactically indistinguishable from one and assigned from the same numbering space.) The IEEE now considers the label MAC-48 to be an obsolete term which was previously used to refer to a specific type of EUI-48 identifier used to address hardware interfaces within existing 802-based networking applications and should not be used in the future. Instead, the term EUI-48 should be used for this purpose.

EUI-64 identifiers are used in:

• FireWire
• IPv6 (as the low-order 64 bits of a unicast network address when temporary addresses are not being used)
• ZigBee / 802.15.4 wireless personal-area networks