发布时间 : 星期二 文章计算机网络第四版习题答案（中文版）更新完毕开始阅读
第 7 章 应用层
7-1 Many business computers have three distinct and worldwide unique identifiers. What are they?
它们是DNS名称，IP地址，以太网地址。
7-2 According to the information given in Fig. 7-3, is little-sister.cs.vu.nl on a class A, B, or C network?
它的IP地址以130开始，所以它在一个B级网络上。
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7-5 DNS uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it solved? DNS 是幂等的，操作可以被重复而不会导致损害。当一个进程发起DNS请求时，它启动一个定时器。当定时器终止时，它仅仅再次发起请求。没有损害会发生。
7-6 In addition to being subject to loss, UDP packets have a maximum length, potentially as low as 576 bytes. What happens when a DNS name to be looked up exceeds this length? Can it be sent in two packets? 这问题不会发生。DNS名字必须短于256 bytes。标准要求如此，因此所有的DNS名称都能适合一个单一的最小长度报文。.
7-7 Can a machine with a single DNS name have multiple IP addresses? How could this occur? 使得，事实上，在图7-3（7.2题），我们能看到一个多重IP地址的例子。记住，IP地址由网络号和主机号组成。如果一台机器有两个以太网卡，它可以处于两个单独的网络上，假如这样的话，它需要两个IP地址。
7-8 Can a computer have two DNS names that fall in different top-level domains? If so, give a plausible example. If not, explain why not. 有可能，www.large-bank.com和www.large-bank.ny.us可能拥有相同的IP地址。因此，一个com域下的项目和一个国家域下的项目无疑是可能的（而且常见）。
7-9 The number of companies with a Web site has grown explosively in recent years. As a result, thousands of companies are registered in the com domain, causing a heavy load on the top-level server for this domain. Suggest a way to alleviate this problem without changing the naming scheme (i.e., without introducing new top-level domain names). It is permitted that your solution requires changes to the client code. 显然有许多方法。一种是把顶级服务器转为服务器外包；另一种是有26台分开的服务器，一台用于以a开头的名称，一台用于b，等等。在引入新服务器后的某些时间周期（比方说3年），旧的服务器仍能继续运行以向人们提供一个适用他们软件的机会。
7-10 Some e-mail systems support a header field Content Return:. It specifies whether the body of a message is to be returned in the event of nondelivery. Does this field belong to the envelope or to the header?
它属于信封，因为分发系统需要知道它的值，用来处理无法递送的电子邮件。
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7-11 Electronic mail systems need directories so people's e-mail addresses can be looked up. To build such directories, names should be broken up into standard components (e.g., first name, last name) to make searching possible. Discuss some problems that must be solved for a worldwide standard to be acceptable.
这远比你能想到的要复杂。以开始而言，全世界一半人先写名字，而后写姓氏。另外一半人（例如中国）则不同。命名系统将不得不辨别任意数量的特定名字，加上姓氏，尽管后者可能具有若干部分，例如 John von Neumann。接着有些人有一个中间首字母，但没有中间名。各种各样的称谓，例如Mr., Miss, Mrs., Ms., Dr., Prof., 或 Lord，可以作为名字的前缀称谓。人们来自不同世代，于是 Jr., Sr., III, IV等等不得不也被包括进来。一些人在名字中使用他们的学术头衔，所以我们需要B.A., B.Sc., M.A., M.Sc., Ph.D，以及其它学位。最后，有些人在名字中列入了奖励和荣誉。例如，英格兰队皇家协会的会员可能加上FRS。现在我们能够接受学者的名字： Prof. Dr. Abigail Barbara Cynthia Doris E. de Vries III, Ph.D., FRS
7-26 A multithreaded Web server is organized as shown in Fig. 7-21. It takes 500 μsec to accept a request and check the cache. Half the time the file is found in the cache and returned immediately. The other half of the time the module has to block for 9 msec while its disk request is queued and processed. How many modules should the server have to keep the CPU busy all the time (assuming the disk is not a bottleneck)?
如果一个模块得到两个请求，平均一个缓存命中一个缓存缺失。总的CPU时间消耗是 1 msec，总的等待时间是 9 msec。这得出一个10%的CPU利用率。所以10个模块能使得CPU保持忙碌。
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7-27 The standard http URL assumes that the Web server is listening on port 80. However, it is possible for a Web server to listen to some other port. Devise a reasonable syntax for a URL accessing a file on a nonstandard port.
正式的Internet标准（草案）RFC 1738方法是 http://dns-name:port/file.
7-28 Although it was not mentioned in the text, an alternative form for a URL is to use the IP address instead of its DNS name. An example of using an IP address is http://192.31.231.66/index.html. How does the browser know whether the name following the scheme is a DNS name or an IP address?
DNS名称不会以数字终止，所以不会有歧义。
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