Networks and Protocols

A network is a collection of computers or computer-like devices that can communicate across a common transmission medium, as shown in Figure 1.1.

Figure 1.1. A typical local network.

In a network, requests and data from one computer pass across the transmission medium (which might be a network cable or a phone line) to another computer. In Figure 1.1, computer A must be able to send a message or request to computer B. Computer B must be able to understand computer A's message and respond to it by sending a message back to computer A.

A computer interacts with the world through one or more applications that perform specific tasks and manage input and output. If that computer is part of a network, some of those applications must be capable of communicating with applications on other network computers. A network protocol is a system of common rules that helps define the complex process of transferring data. The data travels from an application on one computer, through the computer's network hardware, across the transmission medium to the correct destination, and up through the destination computer's network hardware to a receiving application (see Figure 1.2).

Figure 1.2. The role of a network protocol suite.

The protocols of TCP/IP define the network communication process and, more importantly, define how a unit of data should look and what information it should contain so that a receiving computer can interpret the message correctly. TCP/IP and its related protocols form a complete system defining how data should be processed, transmitted, and received on a TCP/IP network. A system of related protocols, such as the TCP/IP protocols, is called a protocol suite.

The actual act of formatting and processing TCP/IP transmissions is performed by a software component known as the vendor's implementation of TCP/IP. For instance, Microsoft TCP/IP is a software component that enables Windows computers to process TCP/IP-formatted data and thus to participate in a TCP/IP network. As you read this book, be aware of the following distinction:

• A TCP/IP standard is a system of rules defining communication on TCP/IP networks.

• A TCP/IP implementation is a software component that performs the functions that enable a computer to participate in a TCP/IP network.

The purpose of the TCP/IP standards is to ensure the compatibility of all TCP/IP implementations regardless of version or vendor.

By the Way

The important distinction between the TCP/IP standards and a TCP/IP implementation is often blurred in popular discussions of TCP/IP, and this is sometimes confusing for readers. For instance, authors often talk about the layers of the TCP/IP model providing services for other layers. In fact, it is not the TCP/IP model that provides services. The TCP/IP model defines the services that should be provided. The vendor software implementations of TCP/IP actually provide these services.