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Tue: 23 Jul 2019


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MAC Addresses

Because MAC addresses are assigned to each interface, rather than to the device itself, some devices will have more than one MAC Address. E.g. Your portable computer may have both a NIC and a Wi-Fi connection and as such will have two MAC Addresses with which to connect to a IEEE 802 network.

A MAC (Media Access Control) addresses (A.K.A Unicast Ethernet Addresses or Global MAC Addresses) is a 48-bit (6-Byte) code represented by 12 HEX digits and are mostly listed as a sequence of 6, 2-digit HEX numbers, e.g. B8:27:EB:DF:36:BC, but can be listed as a sequence of 3, 4-digit numbers as in the case of Cisco: 0000.0C12.3F2C or a 'Broadcast Address': FFFF.FFFF.FFFF The MAC address is a permanent address and forms part of the make-up of the hardware during its manufacture process and will be stored in a ROM chip.

To ensure that each MAC address is unique, the IEEE assign manufactures a 3-Byte OUI (Organisationally Unique Identifier) code. This OUI is then used as a prefix to the MAC address. The manufacture then assigns its own 3-Byte code (one that has never been used in combination with said OUI) as the suffix to the MAC address for any given device, thus making a unique 6-Byte MAC address.

{As an aside, see https://en.wikibooks.org/wiki/Changing_Your_MAC_Address}

Other names used for a MAC Address include:

The term Burned-In Address (BIA) refers to the fact that, as mentioned, a permanent address was 'burned' into a ROM at the time the device was manufactured. The IEEE tend to call MAC addresses Universal Addresses.

The takeaway here is that data sent to a Unicast Address is destined for one MAC address, as opposed to a Multicast Address where data is sent to any device subscribed to that address or a Broadcast Address FF.FF.FF.FF.FF.FF where data is sent to every device on that network.

MAC Address