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Networking Fundamentals · Foundational

Header

Control information a protocol adds in front of the payload, such as addresses, ports, sequence numbers and flags.

Definition

Each layer's header is read by the corresponding layer on the receiving device. Examples include the 20-byte minimum IPv4 header, the 40-byte fixed IPv6 header, the 20-byte minimum TCP header, the 8-byte UDP header and the 14-byte Ethernet header. Header overhead reduces the share of bandwidth available for user data.

Example

An IPv4 header contains the source IP, destination IP, TTL and protocol fields.

Exam tip

UDP's 8-byte header versus TCP's 20-byte minimum is a common comparison point.

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