 of the robustness of the TCP/IP world are based on IP's capability to deliver the data across a wide area of disparate systems and platforms. By using the header, containing the addressing and other pieces of critical information, a trailer is not needed to move the information. The header consists of the following pieces of information (not all are covered): The IP version number of the IP datagram. The header length specifying the datagram header length in units of 32 bit words; the most common is 20 octets. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets X.25, Internets, Intranets, and Extranets Figure 7-27 IPv4 header. Type of Version Service Total Length Identifier Flags Fragment Offset Time to Live Protocol Header Checksum Source Address Destination Address Options and Padding The type of service covers the quality of service and is used when different algorithms are used for routing the datagrams across the network. Unfortunately, the entire industry uses this field differently SO a standard is not defined. The total length defines the length of the entire datagram and overhead. The identification, flags, and offsets control the fragmentation of the datagrams and is used for the reassembly portion of the process. The time-to-live (TTL) is a hop counter that uses a decrementing counter for the delivery of the datagram. How many devices may the datagram pass through before being either delivered or discarded? The Protocol field is used to define the higher-level protocol used (TCP or UDP). The Header checksum is a CRC on the header for error detection of any problems with the datagram. Next, the source and destination addresses show. Options include any special features allowed on the network. Padding is filler used to align the datagram and header to a 32-bit alignment. Downloaded from Digital