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TCP/IP Guide by Charles M. Kozierok

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IP Datagram General Format

Data transmitted over an internetwork using IP is carried in messages called IP datagrams. As is the case with all network protocol messages, IP uses a specific format for its datagrams. Here, I will discuss the IP version 4 (IPv4) datagram format, which was defined in RFC 791 along with the rest of IPv4.

The IPv4 datagram is conceptually divided into two pieces: the header and the payload. The header contains addressing and control fields, while the payload carries the actual data to be sent over the internetwork. Unlike some message formats, IP datagrams do not have a footer following the payload.

Even though IP is a relatively simple, connectionless, and unreliable protocol, the IPv4 header carries a fair bit of information, ...

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