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问题 单项选择题

如下图所示为网络地址转换（NAT）的一个实例。

根据图中信息，标号为④的方格中的内容应为（）。

A．S=135.2.1.1,80 D=202.0.1.1,5001

B．S=135.2.1.1,80 D=192.168.1.1,3342

C．S=202.0.1.1,5001 D=135.2.1.1,80

D．S=192.168.1.1,3342 D=135.2.1.1,80

答案

参考答案：B

解析：

网络地址转换(NAT)设计的基本思路：为每一个公司分配一个或少量的p地址，用于传输Intemet的流量；在公司内部的每一台主机分配一个不能够在Internet上使用的保留的专用IP地址。

如图3-1所示的网络地址转换实例中，如果内部网络地址为192.168.1.1的主机希望访问Internet上地址为135.2.1.1的Web服务器，那么它会产生一个IP地址为192.168.1.1，源端口号为3342，目的IP地址为135.2.1.1，目的端口号为80的IP分组，在图3-1中标记为“S=192.168.1.1,3342 D=135.2.1.1，80”。该IP分组到达执行NAT功能的路由器时，路由器会将分组的源IP地址从内部专用地址转换成可以在外部Internet上路由的全局IP地址。在图3-1中标记为“S=202.0.1.1,5001 D=135.2.1.1,80”。需要注意的是，分组中地址从192.168.1.1转换成202.0.1.16时，传输层客户进程的端口号也需要跟随转换，本例是从3342转换为5001。之所以要同时考虑传输层客户进程端口号的转换，是因为一些应用层的应用程序在执行过程中可能需要在源IP地址与目的IP地址之间除了交换TCP数据之外，还需要交换UDP数据。根据传输层进程通信的规定，TCP与UDP的端口号分配是不同的。那么，TCP与UDP协议的端口号是变化的，因此必须同时考虑IP地址与端口号之间的转换关系。可见，对于内部网到Internet的请求分组，经过NAT转换之后，源IP地址、源端口号发生变化，目的IP地址、目的端口号保持不变。

Web服务器给地址为192.168.1.1的主机返回请求结果时，进入NAT路由器之前的IP分组的源IP地址为135.2.1.1，源端口号为80，目的IP地址为202.0.1.1，目的端口号为5001，即在图3-1中标号为③的方格中的内容应为“S=135.2.1.1，80，D=202.0.1.1,5001”，该IP分组经过查询路由器中NAT转换表可知，目的IP地址202.0.1.1应转换成192.168.1.1，目的端口号5001应转换成3342，而源IP地址、源端口号保持不变。可见，在图3-1中标号为④的方格中的内容应为“S=135.2.1.1,80，D=192.168.1.1,3342”。

单项选择题

电压互感器在额定容量下可以长期运行，但在任何情况下不允许超过最大容量运行，然而可以在工作电压超过额定值（）情况下长期运行。

A、30%

B、20%

C、10%

D、5%
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单项选择题

Passage Two

In 1998 consumers could purchase virtually anything over the Internet. Books, compact discs, and even stocks were available from World Wide Web sites that seemed to spring up almost daily. A few years earlier, some people had predicted that consumers accustomed to shopping in stores would be reluctant to buy things that they could not see or touch in person. For a growing number of time-starved consumers, however, shopping from their home computer was proving to be a convenient alternative to driving to the store.
A research estimated that in 1998 U.S. consumers would purchase ＄ 7.3 billion of goods over the Internet, double the 1997 total. Finding a bargain was getting easier, owing to the rise of online auctions and Web sites that did comparison shopping on the Internet for the best deal.
For all the consumer interest, retailing in cyberspace was still a largely unprofitable business, however. Internet pioneer Amazon. com, which began selling books in 1995 and later branched into recorded music and videos, posted revenue of $153.7 million in the third quarter, up from ＄ 37.9 million in the same period of 1997．Overall, however, the company’s loss widened to ＄ 45.2 million from $ 9.6 million, and analysts did not expect the company to turn a profit until 2003．Despite the great loss, Amazon com had a stock market value of many billions, reflecting investors’ optimism about the future of the industry.
Internet retailing appealed to investors because it provided an efficient means for reaching millions of consumers without having the cost of operating conventional stores with their armies of salespeople. Selling online carried its own risks, however. With so many companies competing for consumers’ attention, price competition was intense and profit margins thin or nonexistent. One video retailer sold the hit movie Titanic for ＄ 9.99, undercutting (削价) the ＄19.99 suggested retail price and losing about ＄ 6 on each copy sold．With Internet retailing still in its initial stage, companies seemed willing to absorb such losses in an attempt to establish a dominant market position.

Finding a bargain on the Internet was getting easier partly because______.

A．there were more and more online auctions

B．there were more and more Internet users

C．the consumers had more money to spend

D．there were more goods available on the Internet
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