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internetworking basics
of the osi model. the osi model represents the building blocks for internetworks. understanding the conceptual model helps you understand the complex pieces that make up an internetwork. 【程序编程相关:Pix515防火墙配置策略实例分析】in the same way that this book provides a foundation for understanding modern networking, this chapter summarizes some common themes presented throughout the remainder of this book. topics include flow control, error checking, and multiplexing, but this chapter focuses mainly on mapping the open system interconnection (osi) model to networking/internetworking functions, and also summarizing the general nature of addressing schemes within the context 【推荐阅读:PPPoE实例:2600路由器接ADSL】an internetwork is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. internetworking refers to the industry, products, and procedures that meet the challenge of creating and administering internetworks. figure 1-1 illustrates some different kinds of network technologies that can be interconnected by routers and other networking devices to create an internetwork. 【扩展信息:实战破解交换机密码】what is an internetwork? figure 1-1: different network technologies can be connected to create an internetwork560)this.style.width=560; onmousewheel = javascript:return big(this) src="/files/uploadimg/20060622/1644440.gif"> history of internetworking the first networks were time-sharing networks that used mainframes and attached terminals. such environments were implemented by both ibms systems network architecture (sna) and digitals network architecture. local-area networks (lans) evolved around the pc revolution. lans enabled multiple users in a relatively small geographical area to exchange files and messages, as well as access shared resources such as file servers and printers. wide-area networks (wans) interconnect lans with geographically dispersed users to create connectivity. some of the technologies used for connecting lans include t1, t3, atm, isdn, adsl, frame relay, radio links, and others. new methods of connecting dispersed lans are appearing everyday. today, high-speed lans and switched internetworks are becoming widely used, largely because they operate at very high speeds and support such high-bandwidth applications as multimedia and videoconferencing. internetworking evolved as a solution to three key problems: isolated lans, duplication of resources, and a lack of network management. isolated lans made electronic communication between different offices or departments impossible. duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did separate support staff. this lack of network management meant that no centralized method of managing and troubleshooting networks existed. internetworking challenges implementing a functional internetwork is no simple task. many challenges must be faced, especially in the areas of connectivity, reliability, network management, and flexibility. each area is key in establishing an efficient and effective internetwork. the challenge when connecting various systems is to support communication among disparate technologies. different sites, for example, may use different types of media operating at varying speeds, or may even include different types of systems that need to communicate. because companies rely heavily on data communication, internetworks must provide a certain level of reliability. this is an unpredictable world, so many large internetworks include redundancy to allow for communication even when problems occur. furthermore, network management must provide centralized support and troubleshooting capabilities in an internetwork. configuration, security, performance, and other issues must be adequately addressed for the internetwork to function smoothly. security within an internetwork is essential. many people think of network security from the perspective of protecting the private network from outside attacks. however, it is just as important to protect the network from internal attacks, especially because most security breaches come from inside. networks must also be secured so that the internal network cannot be used as a tool to attack other external sites. early in the year 2000, many major web sites were the victims of distributed denial of service (ddos) attacks. these attacks were possible because a great number of private networks currently connected with the internet were not properly secured. these private networks were used as tools for the attackers. because nothing in this world is stagnant, internetworks must be flexible enough to change with new demands. open system interconnection reference model the open system interconnection (osi) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. the osi reference model is a conceptual model composed of seven layers, each specifying particular network functions. the model was developed by the international organization for standardization (iso) in 1984, and it is now considered the primary architectural model for intercomputer communications. the osi model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. a task or group of tasks is then assigned to each of the seven osi layers. each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. this enables the solutions offered by one layer to be updated without adversely affecting the other layers. the following list details the seven layers of the open system interconnection (osi) reference model: layer 7—application layer 6—presentation layer 5—session layer 4—transport layer 3—network layer 2—data link layer 1—physical note a handy way to remember the seven layers is the sentence "all people seem to need data processing." the beginning letter of each word corresponds to a layer. all—application layer people—presentation layer seem—sess... 下一页