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Ethernet & MAC addresses
Ethernet is the postal system of the LAN. Every frame carries a destination and source MAC address: 48-bit serial numbers burned into hardware, the first half naming the manufacturer. The diagram shows a frame’s anatomy: addresses, type, payload, and a checksum to catch damage in transit.
Try the MAC/OUI Lookup tool with addresses from your own network: identifying vendors from lease tables is a genuinely useful help-desk trick.