The hardware address of an ethernet card. These are 6 bytes long (eg: 00:00:0C:1A:4B:C3).
You have to 'reserve' part of the MAC address space when you make a network card, so everyone has a unique address. Since everyone's network card is unique, this is often used as a unique identifier for a machine (even though a machine may have multiple network cards, or not have one at all).
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