EMZETT.MAC Addresses
In short: A (theoretically) globally unique hardware address permanently assigned to every network interface (Ethernet, Wi-Fi) — 48 bits, usually written as six hex pairs (AA:BB:CC:11:22:33).
In more detail: Unlike IP addresses, MAC addresses are physically bound to the hardware (layer 2) and normally don’t change when a device switches networks. The first three bytes identify the manufacturer (OUI). Resolving IP to MAC in the local network is handled by ARP.
In Depth
A MAC address consists of 48 bits (6 bytes), usually written as six hex pairs separated by colons or hyphens: 00:1A:2B:3C:4D:5E. The first three bytes (00:1A:2B) form the “organizationally unique identifier” (OUI) and are registered by the manufacturer with the IEEE — this prefix often lets you identify the manufacturer of a network card. The last three bytes are assigned freely by the manufacturer itself and are meant to make every card produced unique.
“Globally unique” only applies in theory: modern operating systems and devices can overwrite MAC addresses in software (“MAC spoofing”) — sometimes for legitimate purposes (privacy when Wi-Fi roaming; many smartphones use random MAC addresses for every new network connection), sometimes to pose as another device. Within a local network segment, switches work with MAC addresses to forward frames specifically only to the right port (instead of sending them to all ports like a hub) — for this, every switch keeps an internal MAC address table.
See also: ARP, IP addresses, Ethernet