问题 单项选择题

用标准访问控制列表禁止非法地址192.168.0.0/16的数据包进出路由器的正确配置是______。

A.access-list 110 deny 192.168.0.0 0.0.255.255
access-list 110 permit any
B.access-list 10 deny 192.168.0.0 255.255.0.0
access-list 10 permit any
C.access-list 50 permit any
access-list 50 deny 192.168.0.0 0.0.255.255
D.access-list 99 deny 192.168.0.0 0.0.255.255
access-list 99 permit any

答案

参考答案:D

解析: 访问控制列表(ACL)的表号(Name)和名字(Number)都是用来标识或引用访问控制列表的。名字用字符串标识,表号用数字表示。标准ACL的表号范围是1~99、1300~1999;扩展ACL的表号范围是100~199、2000~2699。因此,选项A中ACL表号(110)不能满足题意要求。
在路由器全局配置模式下,使用命令access-list access-1ist-number {permit | deny) source wildcard-mask配置标准ACL。其中,访问控制列表通配符(Wildcard-Mask)是子网掩码的反码。本试题中,地址块“192.168.0.0/16”的“/16”是子网掩码255.255.0.0的简化表达形式,其对应的ACL通配符为0.0.255.255。因此选项B的配置语句不符合该命令的语法格式。
ACL默认执行顺序是自上而下的。在配置ACL列表时,要遵循最小特权原则、最靠近受控对象原则,以及默认丢弃原则。其中,最小特权原则是指只给受控对象完成任务所必需的最小的权限,即被控制的总规则是各个规则的交集,只满足部分条件的是不容许通过规则的;最靠近受控对象原则是对所有的网络层访问权限进行控制,也就是说在检查规则时是采用自上而下在ACL中一条条检测的,只要发现符合条件就立刻转发,而不继续检测下面的ACL语句;默认丢弃原则是指在路由交换设备中,默认最后一条ACL语句为deny any any,即丢弃所有不符合条件的数据包。
对于选项C,将“access-list 50 permit any”置于ACL规则的最前面,表示允许所有的数据包进出路由器,则其后的deny语句将不起作用,即不能按照预期的应用需求正确地控制数据包的接收与拒绝。
因此,在Cisco路由器上,用标准访问控制列表禁止非法地址192.168.0.0/16的数据包进出路由器,可能的一种配置语句,如下所示:
Router (conflg)# access-list 99 deny 192.168.0.0 0.0.255.255
Router (conflg)# access-list 99 permit any

阅读理解

PITTSBURGH – For most people, snakes seem unpleasant or even threatening. But Howie Choset sees in their delicate movements a way to save lives.

The 37-year-old Carnegie Mellon University professor has spent years developing snake-like robots he hopes will eventually slide through fallen buildings in search of victims trapped after natural disasters or other emergencies.

Dan Kara is president of Robotics Trends, a Northboro, Mass.-based company that publishes an online industry magazine and runs robotics trade shows. He said there are other snake-like robots being developed, mainly at universities, but didn’t know of one that could climb pipes.

The Carnegie Mellon machines are designed to carry cameras and electronic sensors and can be controlled with a joystick(操纵杆). They move smoothly with the help of small electric motors, or servos, commonly used by hobbyists in model airplanes.

Built from lightweight materials, the robots are about the size of a human arm or smaller. They can sense which way is up, but are only as good as their human operators, Choset added.

Sam Stover, a search term manager with the Federal Emergency Management Agency based in Indiana, said snake-type robots would offer greater mobility than equipment currently available, such as cameras attached to extendable roles.

“It just allows us to do something we’ve not been able to do before,” Stover said, “We needed them yesterday.”

He said snifter dogs are still the best search tool for rescue workers, but that they can only be used effectively when workers have access to damaged building.

Stover, among the rescue workers who handled the aftermath (后果) of Hurricane Katrina, said snake robots would have helped rescuers search flooded houses in that disaster.

Choset said the robots may not be ready for use for another five to ten years, depending on funding.  

小题1:Which institution is responsible for the development of Choset’s robots?

A.Robotics Trends.

B.Pittsburgh City Council.

C.Carnegie Mellon University.

D.Federal Emergency Management Agency.小题2:Choset believes that his invention ______.

A.can be attached to an electronic arm

B.can be used by hobbyists in model airplanes

C.can find victims more quickly than a snifter dog

D.can sense its way no better than its operators小题3:By saying “We needed them yesterday” (paragraph 7), Stover means that snake-like robots _____.

A.could help handle the aftermath of Hurricane Katrina

B.would have been put to use in past rescue work

C.helped rescuers search flooded houses yesterday

D.were in greater need yesterday than today小题4:What is the text mainly about?

A.Snake-like robots used in industries.

B.Snake-like robots made to aid in rescues.

C.The development of snake-like robots.

D.The working principles of snake-like robots.

单项选择题