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The TCP/IP protocol Datagram Format

The TCP/IP protocols were built to transmit data over the ARPAnet, which was a packet-switching network. A packet is a block of data that carries with it the information necessary to deliver it, similar to a postal letter, which has an address written on its envelope. A packet-switching network uses the addressing information in the packets to switch packets from one physical network to another, moving them toward their final destination. Each packet travels the network independently of any other packet.

The datagram is the packet format defined by the Internet Protocol. Figure 1-5 is a pictorial representation of an IP datagram. The first five or six 32-bit words of the datagram are control information called the header. By default, the header is five words long; the sixth word is optional. Because the header's length is variable, it includes a field called Internet Header Length (IHL) that indicates the header's length in words. The header contains all the information necessary to deliver the packet.

Internet Protocol packet format

The Internet Protocol delivers the datagram by checking the Destination Address in word 5 of the header. The Destination Address is a standard 32-bit IP address that identifies the destination network and the specific host on that network. (The format of IP addresses is explained in Chapter 2.) If the Destination Address is the address of a host on the local network, the packet is delivered directly to the destination. If the Destination Address is not on the local network, the packet is passed to a gateway for delivery. Gateways are devices that switch packets between the different physical networks. Deciding which gateway to use is called routing. IP makes the routing decision for each individual packet.

The above is an excerpt from: TCP/IP Network Administration (3rd Edition; O'Reilly Networking)

Reader G. Maxwell says,"As with all of O'Reilly's books, this one is technically accurate and fundamentally sound. It does not teach TCP/IP from a simplistic approach - telling you only what you need to know and leaving you begging for more. It lays a ground work based upon the actual theory of these protocols and how they were developed and the thinking that was involved in their creation. From there, it takes you step by step through the layers of the protocols and presents everything that most people would need to know--even more than they would need to know. Especially enlightening were the chapters on IPv6 - the next generation of the IP protocol, and the chapter covering subnetting. Overall, if you need the one book to explain TCP/IP and the "ins-and-outs" of these networking protocols, look no further. This book has all you'll need.

The ARPANET (Advanced Research Projects Agency Network) was an early implementation of a packet switching network.

Currently the Internet is changing form IPv4, which uses 32-bit (4-byte) addresses to IPv6 which uses 128-bit (16-byte) addresses, so the datagram format will change. Both IPv4 and IPv6 packets have a maximum length of 64 kB.

More Networking Protocols and Standards:
• Remote Control Protocols
• IP Addressing
• IPv6 Prefix Length Notation
• Video - Transport Layer (Layer 4) of OSI Networking Model
• TCP/IP Protocol Suite
• The TCP/IP protocol Datagram Format
• X.25 and Frame Relay Overview
• Protocol Suites
• WAN Network Protocols - DSL, SONET, HDLC, DWDM, DLSW+
• IPv4 to IPv6 Transition With the Dual-Stack Technique

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