问题 单项选择题

一个IP数据包经过一台路由器转发到另一个网络,该IP数据包的头部字段中一定会发生变化的是______。

A.源IP
B.协议号
C.目的IP
D.TTL

答案

参考答案:D

解析: 本题考查路由器的工作原理和IP分组中TTL字段的含义。
1.IP数据包结构
IP数据包是Internet的基本传送单元,包括数据包包头和数据区两部分。下图表示了IP数据包格式。


IP数据包格式
IP协议的数据包头中主要字段如下:
·版本字段
4bit。用来标识IP协议的版本。目前的IP协议版本是4,下一代IP (IPv6)协议为6。
·包头长度字段
4bit。该字段紧跟在版本号字段后,表示以32位(4个字节)为单位的包头长度。
·服务类型字段
8bit。指明服务类型或优先级,用于实现区分服务或优先级选路机制。
·总长度字段
16bit。以字节为单位的IP包长度(包含IP头在内),IP包最大长度65535字节。
·标识符与分段偏移量字段
IP包可能会被分段,这些字段用于分段和到达目的地后的重组。
·协议字段
协议字段指出用于IP数据包携带的高层协议。IP协议的高层最常用的是TCP和UDP;TCP的协议代码为6;UDP协议代码为17。
·源地址和目的地址字段
源地址字段和目的地址字段都是32位(32bit)。源地址字段存放发送该IPv4数据包的原始IPv4地址(数据包会经路由器转发,转发路由器地址不是源地址);目的地址字段存放最终接收该IPv4数据包的设备的IP地址(转发路由器也会接收其他路由器转发过来的IPv4包,但目的地址并不指向该转发路由器)。
2.路由器转发原理
从OSI七层模型的角度看,路由器是工作在三层(网络层),完成三层协议转发的设备。对Internet来说,其三层协议就是IP协议,因此Intemet的路由器可以称为IP路由器。
在Internet中,路由器用于连接多个逻辑上分开的网络,这些逻辑网络是指一个单独的网络或一个子网,用网络ID来标识。当数据从一个子网传输到另一个子网时,必须通过路由器转发来完成。路由器具有判断网络地址和选择路径(路由选择)的功能。
总体而言,Internet路由器是连接IP子网的设备,并在两个或多个不同的IP子网之间进行IP包的转发。IP子网由IP地址中网络ID(包括子网ID)部分来标识,网络ID不同即为不同的IP子网。
3.TTL字段的作用
TTL是Time To Live的缩写,含义为“生存时间”。为避免因错误选路而产生的环路现象(IP包在一个循环的路由中传递,永远到达不了目的地)。该字段定义了IP数据包可存在的最大期限。由于确切的生存时间很难把握,该字段通常用跳跃站点数来度量,即当数据包从一个网络传送到另一个网络时(即经过一个路由器的转发),该字段的值减1。当该字段为0时,数据包将被丢弃。

判断题
单项选择题

Many will know that the word "muscle" comes from the Latin for "mouse" (rippling under the skin, so to speak ). But what about "chagrin", derived from the Turkish for roughened leather, or scaly sharkskin. Or "lens" which comes from the Latin "lentil" or "window" meaning "eye of wind" in old Norse Looked at closely, the language comes apart in images, like those strange paintings by Giuseppe Arcimboldo where heads are made of fruit and vegetables.

Not that Henry Hitchings’s book is about verbal surrealism. That is an extra pleasure in a book which is really about the way the English language has roamed the world helping itself liberally to words, absorbing them, forgetting where they came from, and moving on with an ever-growing load of exotics, crossbreeds and subtly shaded near-synonyms. It is also about migrations within the language’s own borders, about upward and downward mobility, about words losing their roots, turning up in new surroundings, or lying in wait, like "duvet" which was mentioned by Samuel Johnson, for their moment.

All this is another way of writing history. The Arab etymologies of " saffron ", "crimson" and "sugar" speak of England’s medieval trade with the Arab world. We have "cheque" and "tariff" from this source too, plus "arithmetic" and "algorithm"-just as we have "etch" and "sketch" from the Dutch, musical terms from the Italians and philosophical ones from the Germans. French nuance and finesse are everywhere. At every stage, the book is about people and ideas on the move, about invasion, refugees, immigrants, traders, colonists and explorers.

This is a huge subject and one that is almost bound to provoke question-marks and explosions in the margins-soon forgotten in the book’s sheer sweep and scale. A balance between straight history and word history is sometimes difficult to strike, though. There is a feeling, occasionally, of being bundled too fast through complex linguistic developments and usages, or of being given interesting slices of history for the sake, after all, of not much more than a "gong" or a "moccasin". But it is churlish to carp. The author’s zest and grasp are wonderful. He makes you want to check out everything-" carp" and "zest" included. Whatever is hybrid, fluid and unpoliced about English delights him.

English has never had its Acad mie Francaise, but over the centuries it has not lacked furious defenders against foreign "corruption". There have been rearguard actions to preserve its "manly" pre-Norman origins, even to reconstruct it along Anglo-Saxon lines: "wheel- saddle" for bicycle, "painlore" for pathology. But the omnivorous beast is rampant still. More people speak it as their second language than as their first. Forget the language of Shakespeare. It’s "Globish" now, the language of aspiration. No one owns it, a cause for despair to some. Mr. Hitchings admits to wincing occasionally, but almost on principle he is more cheerful than not.

According to the text, which of the following is TRUE ?()

A."Muscle" derives from Italian

B."Chagrin" derives from Turkish

C."Crimson" derives from Persian

D."Sketch" derives from German