Although the computer industry is still young compared to other industries (e.g., automobiles and air transportation), computers have made spectacular progress in a short time. During the first two decades of their existence, computer systems were highly centralized, usually within a single large room. Not infrequently, this room had glass walls, through which visitors could gawk at the great electronic wonder inside. A medium-sized company or university might have had one or two computers, while large institutions had at most a few dozen. The idea that within twenty years equally powerful computers smaller than postage stamps would be mass produced by the millions was pure science fiction.

与汽车、航空运输等其他行业相比，尽管计算机仍是朝阳产业，但是在短短的时间内却取得了惊人的进展。在头二十年的发展过程中，计算机系统高度集中化，通常在一个很大的房间内。一般这个房间有玻璃墙，透过玻璃墙，参观的人会被其中巨大奇异的电子设备所吸引。中等企业或是大学可能拥有一两台计算机，而大型的研究院最多几十台。在二十年里，让数百万的企业大规模生产比邮票还小的强大计算机的想法纯粹是科学幻想。（要在二十年内生产出大量同样功能但体积比邮票还小的计算机，在当时的人们看来纯属科学幻想）（虚拟！）

The merging of computers and communications has had a profound influence on the way computer systems are organized. The concept of the ''computer center'' as a room with a large computer to which users bring their work for processing is now totally obsolete. The old model of a single computer serving all of the organization's computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computer networks. The design and organization of these networks are the subjects of this book.

计算机和通信的融合已经对计算机系统的组织方式产生了深远的影响。以前人们心中“计算机中心”的概念已完全过时了，即用户带着他们的任务到一间放大型计算机的房间内处理。那种由一台计算机为所有机构的计算需要提供服务的旧模式已经被大量独立而又相互联系的计算机所取代。这些系统就是计算机网络。而这些网络的设计和组织便是该书的主要内容。

Throughout the book we will use the term ''computer network'' to mean a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used. Networks come in many sizes, shapes and forms, as we will see later. Although it may sound strange to some people, neither the Internet nor the World Wide Web is a computer network. By the end of this book, it should be clear why. The quick answer is: the Internet is not a single network but a network of networks and the Web is a distributed system that runs on top of the Internet.

书中，我们将用术语“计算机网络”来表示由一种技术相互联系起来的自主计算机的集合。两台计算机如果能相互交换信息，称作互联。这种连接不需要通过铜线；光纤，微波，红外线和通信卫星等都可以用来建立连接。后面我们可以看到，网络可以有不同的大小，形状，形式。尽管对很多人来说比较陌生，互联网和万维网都不是计算机网络（可能很多人对此会觉得很奇怪）。在本书的结尾部分，我们将详细阐述其原因。概言之（现在给出一个简单的答案），互联网不是单一的网络，而是许多网络联系在一起构成的一个网络；万维网是在互联网之上运行的分布式系统。

There is considerable confusion in the literature between a computer network and a distributed system. The key distinction is that in a distributed system, a collection of independent computers appears to its users as a single coherent system. Usually, it has a single model or paradigm that it presents to the users. Often a layer of software on top of the operating system, called middleware, is responsible for implementing this model. A well-known example of a distributed system is the World Wide Web, in which everything looks like a document (Web page).

在一些文献中，计算机网络和分布式系统是比较容易混淆的概念。他们的主要区别在于，分布式系统中，相互独立的计算机组合对用户来说是一个单一的关联系统。（在一个分布式系统中，一组独立的计算机展现给用户的是一个统一的整体，就好像一个系统似的）通常，呈现给用户的只有一个模式或者范例。位于操作系统之上的一层软件称作中间件（middleware），负责实施该模式。分布式系统的一个众所周知的例子是万维网，即在万维网中，所有的内容看上去像文档（网页）一样。

In a computer network, this coherence, model, and software are absent. Users are exposed to the actual machines, without any attempt by the system to make the machines look and act in a coherent way. If the machines have different hardware and different operating systems, that is fully visible to the users. If a user wants to run a program on a remote machine, he [ ] has to log onto that machine and run it there.

[ ] ''He'' should be read as ''he or she'' throughout this book.

 在计算机网络中，这种关联、模式和软件都是不存在的。用户面对的是实实在在的机器，而不能通过系统使机器的样子和行为一致。如果机器有不同的硬件和操作系统，那么这些差异对用户来说是完全可见的。如果用户想运行一台远程计算机上的程序，他可以登录到那台机器并运行程序。（本书中，他可以看作“他”或“她”）

In effect, a distributed system is a software system built on top of a network. The software gives it a high degree of cohesiveness and transparency. Thus, the distinction between a network and a distributed system lies with the software (especially the operating system), rather than with the hardware.

实际上，分布式系统是建立在网络之上的软件系统。该软件提供了高度的粘聚性和透明度。因此，网络与分布式系统之间的区别在于软件（特别是操作系统），而不是硬件。

Nevertheless, there is considerable overlap between the two subjects. For example, both distributed systems and computer networks need to move files around. The difference lies in who invokes the movement, the system or the user. Although this book primarily focuses on networks, many of the topics are also important in distributed systems. For more information about distributed systems, see (Tanenbaum and Van Steen, 2002).
尽管如此，分布式系统和计算机网络之间还是有很多重叠的地方。比如，他们都需要移动文档。差异在于谁发起的移动，是系统还是用户。虽然本书大部分在关注网络，但在分布式系统中，许多主题同样非常重要。关于分布式系统的更多信息，请参看 Tanenbaum and Van Steen, 2002 。