an Ethernet frame

Think of an Ethernet frame as a single addressed envelope that one machine drops onto the local network. Everything a host sends over a wired LAN — a web request, a file chunk, a video packet — gets wrapped in one of these envelopes before it leaves the network card. The frame is the unit the link layer actually delivers.

The envelope has fixed parts in a fixed order. First a preamble (a repeating pattern of bits, ending in a start-of-frame marker) lets the receiver's clock lock onto the incoming bits. Then comes the 6-byte destination MAC address, the 6-byte source MAC address, and a 2-byte type/length field that says what is inside (for example, the value for an IPv4 packet). Next is the payload — the actual data, between 46 and 1500 bytes (the standard 1500-byte limit is the Ethernet MTU). Finally a 4-byte CRC (the frame check sequence) lets the receiver detect if any bits got corrupted in transit; a damaged frame is silently dropped.

Because the addresses sit right at the front, a switch can read the destination MAC and decide where to send the frame without ever looking at the payload. The CRC only detects errors, it does not fix them — Ethernet leaves recovery to higher layers like TCP. And the 1500-byte payload limit is why a large file is chopped into many frames rather than sent as one.