By 2001, there were approximately 400 million Internet users worldwide, up from 100 million users at year-end 1997. That figure is projected to continue to grow to close to 800 million by 2003. One would think that with such incredible growth, the Internet would be overloaded. However, this has not been true for several reasons. First, client/server computing is highly extensible: By simply adding servers and clients, the population of Internet users can grow indefinitely. Second, the Internet architecture is built in layers so that each layer can change without disturbing developments in other layers. For instance, the technology used to move messages through the Internet can go through radical changes to make service faster without being disruptive to your desktop applications running on the Internet.
截至2001年，全世界因特网用户从1997年底的1亿上升到大约有4亿。这个数字计划到2003年继续增长到8亿。有人可能会想：以如此令人难以置信的速度增长,因特网会超载。然而，由于几个原因这没有成为现实。首先，客户机/ 服务器 计算技术可以高度扩展：只需增加服务器和客户机,因特网用户人口能无限增长。其次，因特网是按照层次结构建造的，因此每一层能在不干扰其他层发展的前提下改变。举例来说，用于在因特网上移动信息的技术可以实现根本的改变使服务更快，而不会中断你的台式机在因特网上运行的应用。
The Internet can be viewed conceptually as having four layers: the network technology substrate, transport services and representation standards, middleware services, and applications. The network technology substrate is composed of telecommunications networks and protocols. The transport layer houses the TCP/IP protocol. The applications layer contains client applications such as the World Wide Web , e-mail, and audio or video playback. Middleware is the glue that ties the applications to the communications networks, and includes such services as security, authentication, addresses, and storage repositories.
因特网能被概念性地分为四层： 网络技术 基体、传送服务和表示 标准 、中间件服务和应用。网络技术基体由电信网络和协议组成。传送层收容TCP/IP协议。应用层包含客户机的应用，例如万维网、电子邮件和声频或 视频 回放。中间件是将应用与通信网络连接起来的粘合剂,并且包括如 安全 、验证、地址和 存储 等服务。
Users work with applications (such as e-mail) and rarely become aware of middleware that operates in the background. Because all layers use TCP/IP and other common standards linking all four layers, it is possible for there to be significant changes in the network layer without forcing changes in the applications layer. The network layer is described below.
用户通过应用程序(例如电子邮件)工作，很少知道中间件在后台运行。因为各层使用TCP/IP协议和其他公共标准联结所有四层,所以在网络层发生重大变化而不强迫应用层变化是可能的。网络层在下面描述。
