What is a MAC-address?

A (Media Access Control) identifies a network adapter, such as the Ethernet card in your computer.

The number is 48 bits or 64 bits long and is in hexadecimal notation, represented byte for byte. The first 24 bit carry a description of the manufacturer, the other 24 bits or 40 bits carry a manufacturer-numbering (or serial). Its usage is described in RFC 5342.

Variants

MAC-48

The MAC-48 ID is 48 bits long, which accounts for 24 bits for the producer (the so-called OUI, organizationally unique identifier) and 24 bits for numbering the manufacturer devices.

Presentation:

MAC-48: 01-23-45-67-89-ab 
        producer-device--
      = 01:23:45:67:89:ab 
      = 0123.4567.89ab

MAC-64

The MAC-64 ID is 64 bits long, which accounts for 24 bit for the manufacturer and each manufacturer has 40 bits for numbering its devices.

Presentation:

MAC-64: 01-23-45-67-89-ab-cd-ef 
        producer-device--------
      = 01:23:45:67:89:ab:cd:ef
      = 0123.4567.89ab.cdef

Address assignment

UAA addresses can be (universally admistered adresses) order LAA (locally administered addresses).

Universally administered address

The UAA (Universally Administered Address) is unique and assigned to the device by its manufacturer. They can not be changed and therefore is also called the BIA (burned in address).

The complete ID is OUI-UAA, with 24 bits for the organizationally unique identifier of the manufacturer and 24 bits or 40 bits for the UAA of the device from that manufacturer.

A MAC address identifies itself as assigned by the manufacturer that bit 1 in the first byte is cleared (thats the binary one in the first byte of the OUI).

Locally administered address

The LAA (locally administered address) is assigned by the network administrator to override the address of the manufacturer and thus allows the network-internal change of the MAC address. As self-assigned, LAA do not describe a any manufacturer.

An address identifies himself as a LAA in that bit 1 is set in the first byte.

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