In computer networking, a Media Access Control address (MAC address) is a unique identifier assigned to most network adapters or network interface cards (NICs) by the manufacturer for identification, and used in the Media Access Control protocol sub-layer. If assigned by the manufacturer, a MAC address usually encodes the manufacturer’s registered identification number. It may also be known as an Ethernet Hardware Address (EHA), hardware address, adapter address, or physical address.

There are three numbering spaces, managed by the Institute of Electrical and Electronics Engineers (IEEE), which are in common use for formulating a MAC address: MAC-48, EUI-48, and EUI-64. The IEEE claims trademarks on the names “EUI-48” and “EUI-64”, where “EUI” stands for Extended Unique Identifier.

Although intended to be a permanent and globally unique identification, it is possible to change the MAC address on most of today’s hardware, an action often referred to as MAC spoofing. Unlike IP address spoofing, where a sender spoofing their address in a request tricks the other party into sending the response elsewhere, in MAC address spoofing (which takes place only within a local area network), the response is received by the spoofing party.

A host cannot determine from the MAC address of another host whether that host is on the same OSI Layer 2 network segment as the sending host, or on a network segment bridged to that network segment.

In TCP/IP networks, the MAC address of a subnet interface can be queried with the IP address using the Address Resolution Protocol (ARP) for Internet Protocol Version 4 (IPv4) or the Neighbor Discovery Protocol (NDP) for IPv6. On broadcast networks, such as Ethernet, the MAC address uniquely identifies each node and allows frames to be marked for specific hosts. It thus forms the basis of most of the Link layer (OSI Layer 2) networking upon which upper layer protocols rely to produce complex, functioning networks.
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