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The TCP/IP Guide

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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)

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IP CIDR Addressing Example
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IP Datagram Encapsulation
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IP Datagram Encapsulation and Formatting

The primary job of the Internet Protocol is the delivery of data between devices over an internetwork. On its journey between two hosts in an internet, this data may travel across many physical networks. To help ensure that the data is sent and received properly, it is encapsulated within a message called an IP datagram. This datagram includes several fields that help manage the operation of IP and ensure that data gets where it needs to go.

In this section I take a look at how the Internet Protocol takes data passed to it from higher layers and packages it for transmission. I begin with a general discussion of IP datagrams and encapsulation. I then describe the general format of IP datagrams, including the fields used in the IP header and how they are interpreted. I also include a brief discussion of IP datagram options and their use.

Background Information: this section assumes at least passing familiarity with IP addressing concepts. It also makes reference to the section on datagram fragmentation and reassembly.


Note: IP datagrams are sometimes called IP packets. Whether “datagram” or “packet” is the preferred term seems to depend on whom you ask; even the standards don’t use one term exclusively. On the other hand, I have seen IP datagrams called IP frames, and that’s definitely not correct! The topic on messages and names in the fundamentals chapter describes these terms more completely.


Quick navigation to subsections and regular topics in this section



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IP Datagram Encapsulation
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Version 3.0 - Version Date: September 20, 2005

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