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In computer networking a Media Access Control address (MAC address) or Ethernet Hardware Address (EHA) or hardware address or adapter address is a quasi-unique identifier attached to most network adapters (NICs). It is a number that acts like a name for a particular network adapter, so, for example, the network cards (or built-in network adapters) in two different computers will have different names, or MAC addresses, as would an Ethernet adapter and a wireless adapter in the same computer, and as would multiple network cards in a router. However, it is possible to change the MAC address on most of today's hardware.

Most layer 2 network protocols use one of three numbering spaces managed by the IEEE: MAC-48, EUI-48, and EUI-64, which are designed to be globally unique. Not all communications protocols use MAC addresses, and not all protocols require globally unique identifiers. The IEEE claims trademarks on the names "EUI-48" and "EUI-64". (The "EUI" stands for Extended Unique Identifier.)

MAC addresses, unlike IP addresses and IPX addresses, are not divided into "host" and "network" portions, so a host cannot determine, from the MAC address of another host, whether that host is on the same layer 2 network segment as the sending host or a network segment bridged to that network segment and, if it's not, cannot determine the MAC address of a router that is on the same network segment as the sending host or a segment bridged to that network segment and that can help route the packet to the destination host.

ARP is commonly used to convert from addresses in a layer 3 protocol such as Internet Protocol (IP) to the layer 2 MAC address. On broadcast networks, such as Ethernet, the MAC address allows each host to be uniquely identified and allows frames to be marked for specific hosts. It thus forms the basis of most of the layer 2 networking upon which higher OSI Layer protocols are built to produce complex, functioning networks.