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Often called a physical address (PHY addr), the Media Access Control address (MAC address) is a unique identifier assigned to each Network Interface Controller (NIC). Typically this address is “burned” into the ROM of the NIC hardware, though it may be changed via software.
MAC Addresses are formed by six sets of two hexadecimal digits (octets), typically separated by a dash (-) or colon (:) and may be either universally or locally administered. For example, 00:C0:CA:8F:5E:80.
Universally administered MAC addresses are unique to each NIC manufacturer. The first three octets represent the manufacturer or vendor as its Organizationally Unique Identifier (OUI). In the example above, 00:C0:CA represents the OUI for ALFA, INC – a popular Taiwanese WiFi equipment maker. OUIs are assigned by the Institute of Electrical and Electronics Engineers, Incorporated (IEEE). The vendor of any particular OUI may be determined by checking the IEEE MAC database, or the Wireshark OUI Lookup Tool.
Locally administered MAC addresses are typically assigned by the network administrator, replacing the universally administered address burned into ROM. For example, one may set their MAC address to DE:AD:BE:EF:C0:FE. This is sometimes considered MAC spoofing.