A MAC address is a 12-character hexadecimal string divided into six "octets" (e.g., 00:1A:2B:3C:4D:5E ).
The first octet contains two special bits. The least significant bit is the , which must be 0 for unicast (device-specific) communication. The second least significant bit is the U/L bit (Universally/Locally administered) . When this bit is 0 , it indicates a globally unique address burned into the hardware. When this bit is 1 , it marks the address as "locally administered" —meaning a system administrator or user has assigned it. A MAC address is a 12-character hexadecimal string
The first octet of a locally administered MAC address must be in the form of x2 , x6 , xA , or xE in hexadecimal (where x is any hex digit). Acceptable examples: 02 , 06 , 0A , 0E , 12 , 16 , 1A , 1E , 22 , 26 , 2A , 2E , etc. The second least significant bit is the U/L
: The Organizationally Unique Identifier (OUI) assigned to hardware vendors by the IEEE. The first octet of a locally administered MAC
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Within this first octet, the second least significant bit is known as the . This bit tells network devices whether the MAC address is universally administered (which would mean it is a factory-assigned, globally unique address) or locally administered (which means it is a custom address assigned by a user or administrator).