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Networking Fundamentals · Intermediate

IPv4 Header

The 20- to 60-byte header at the front of every IPv4 packet carrying addressing and control fields.

Definition

Fields include Version, Internet Header Length, DSCP and ECN (formerly Type of Service), Total Length, Identification, Flags, Fragment Offset, Time to Live, Protocol, Header Checksum, Source Address, Destination Address and optional Options. The header checksum must be recomputed at every hop because TTL changes.

Example

A packet capture shows Protocol: TCP (6) and TTL: 64 inside the IPv4 header.

Exam tip

Minimum IPv4 header is 20 bytes; the IPv6 fixed header is 40 bytes but has fewer fields.

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