Term Description
header header refers to the first part of a data cell or packet, containing such information as source and destination addresses, and instructions on how the telecommunications network is to handle the data. The header is part of the overhead in a data transmission protocol.

For typical TCP/IP transmissions (most Internet traffic), the header is usually 40 bytes of each packet (20-byte TCP and 20-byte IP headers). Note TCP and IP headers can be larger than 20 bytes if "options" are enabled. ICMP headers (as in pings) are 28 bytes. UDP headers are 8 bytes.
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