Internet Core Protocols: The Definitive Guide

The IP Header

IP datagrams consist of two basic components: an IP header that dictates how the datagram is treated and a body part that contains whatever data is being passed between the source and destination systems.

An IP datagram is made up of at least thirteen fields, with twelve fields being used for the IP header, and one field being used for data. In addition, there are also a variety of supplemental fields that may show up as “options” in the header. The total size of the datagram will vary according to the size of the data and the options in use.

Table 2.11 lists all of the mandatory fields in an IP header, along with their size (in bits) and some usage notes. For more detailed descriptions of these fields, refer to the individual sections throughout this chapter.

Table 2.11. The Fields in an IP Datagram

Field	Bits	Usage Notes
Version	4	Identifies the version of IP used to create the datagram. Every device that touches this datagram must support the version shown in this field. Most TCP/IP products use IP v4. NOTE: This book only covers IP v4.
Header Length	4	Specifies the length of the IP header in 32-bit multiples. Since almost all IP headers are 20 bytes long, the value of this field is almost always 5 (5 × 32 = 160 bits, or 20 bytes).
Type-of-Service Flags	8	Provide a prioritization service to applications, hosts, and routers on the Internet. By setting the appropriate flags in this field, an application could request that the datagram be given higher priority than others waiting ...