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Networking BasicsNetworking BasicsLike any other technical subject, networking is full of terminology and jargon that a newcomer might find daunting. This section introduces some basic concepts of networking, starting with a layered model of networking and proceeding to details of Ethernet and TCP/IP network protocols. The OSI Seven-Layer ModelA widely used conceptual model of networking is the seven-layer Open Systems Interconnection (OSI) reference model, developed by the International Organizaton for Standardization (ISO). The OSI reference model describes the flow of data between the physical connection to the network and the end-user application. Each layer is responsible for providing particular functionality, as shown in Figure 6-1. 
Figure 6-1: The OSI Seven-Layer Reference Model of Networking. As Figure 6-1 shows, the OSI layers are numbered from bottom to top. Basic functions, such as physically sending data bits through the network cable, are at the bottom; functions that deal with higher-level abstractions of the data are at the top. The purpose of each layer is to provide services to the next-higher layer in a manner such that the higher layer does not have to know how the services are actually implemented. In fact, each layer is designed so that it does not have to know how the other layers work. The purposes of the seven layers in the OSI reference model are as follows:
The OSI model is not specific to any hardware or software; it simply provides an architectural framework and gives us a common terminology for discussing various networking capabilities. A Simplified Four-Layer TCP/IP Network ModelThe OSI seven-layer model is not a specification; it provides guidelines for organizing all network services. Most implementations adopt a layered model for networking services, and these layered models can be mapped to the OSI reference model. The TCP/IP networking model, for example, can be adequately represented by a simplified model. Network-aware applications usually deal with the top three layers (session, presentation, and application) of the OSI seven-layer reference model. Thus, these three layers can be combined into a single layer called the application layer. The bottom two layers of the OSI model—physical and data link—also can be combined into a single physical layer. These combinations result in a simplified four-layer model, as shown in Figure 6-2. 
Figure 6-2: A Simplified Four-Layer TCP/IP Networking Model. At each of these layers, information is exchanged through one of many network protocols. Network ProtocolsA network protocol refers to a detailed process the sender and receiver agree upon for exchanging data at a specific layer of the networking model. Thus, you find the following protocols in the simplified four-layer network model of Figure 6-2:
The term protocol suite refers to a collection of two or more protocols from these layers that form the basis of a network. The following are some of the well-known protocol suites:
Of these protocol suites, you are probably most interested in the TCP/IP protocol suite, because that’s what Linux and other UNIX systems support well.
Learning More about TCP/IPThis chapter gives you an overview of TCP/IP and Ethernet networking, then moves on to Linux-specific instructions for setting up TCP/IP networking. A single chapter simply isn’t enough to provide all available information about TCP/IP. For more information on TCP/IP, consult one of the following books:
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