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(MTU) The largest number of bytes of "payload" data a frame can carry, not counting the frame's header and trailer.
A frame is a single unit of transportation on the data link layer. It consists of header data plus data which was passed down from the network layer (e.g. an IP datagram) plus sometimes trailer data.
An Ethernet (V2) frame has a MTU of 1500 bytes but the size of the frame can be up to 1526 bytes (22 byte header, 4 byte CRC trailer).
See also fragmentation.