CCNA640-802题库解析–基本网络命令定义与作用

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本文主要是结合例题讲解基础网络命令的定义和作用（Ping,Arp,ipconfig,telnettraceroute等）
For security reasons, thenetworkadministrator needs to prevent pings into the corporate networks from hosts outside the internetwork. Which protocol should be blocked with access control lists?
A: IP
B: ICMP
C: TCP
D: UDP
Correct Answers:B
PING命令利用ICMP协议的echo,和echo-replay两个报文来检测链路是否连通的，它所产生的数据报文是IP网络中能够生成和寻址的最小报文，通常是用来网络中的排错测试。所以如果要阻止PING的流量到网络，就只要过滤掉ICMP的应用就可以了。
What is the purpose of an ARP request message?
A: It binds the IP address of a host to the network that it is on.
B: It builds a correlation between an IP address and a MAC address.
C: It provides connectivity and path selection between hosts on a network.
D: It encapsulates the Layer 3 address and then passes the packet to Layer 2.
E: It creates a session by passing a header with the destination Layer 2 address to the transport layer.
Correct Answers:B
ARP是地址解析协议，用来将IP地址映射到MAC地址，以便设备能在多路访问介质上通信。比如IP地址为192.168.0.1网卡MAC地址为00-03-0F-FD-1D-2B。整个转换过程是一台主机先向目标主机发送包含IP地址信息的广播数据包，即ARP请求，然后目标主机向该主机发送一个含有IP地址和MAC地址数据包，通过MAC地址两个主机就可以实现数据传输了。
The user of Host1 wants to ping the DSL modem/router at 192.168.1.254. Based on the Host1 ARP table that is shown in the exhibit, what will Host1 do?
A: send a unicast ARP packet to the DSL modem/router
B: send unicast ICMP packets to the DSL modem/router
C: send Layer 3 broadcast packets to which the DSL modem/router responds
D: send a Layer 2 broadcast that is received by Host2, theswitch, and the DSL modem/router
Correct Answers:B
在上面的表中我们可以看到ARP表中有关于192.168.1.254的ARP条目，所以在这主机都只需要发送单播的ICMP包到DSL modem/router即可。
What are two characteristics of Telnet? (Choose two.)
A:It sends data in clear text format.
B:It is no longer supported onCisconetwork devices.
C:It is more secure than SSH.
D:It requires an enterprise license in order to be implemented.
E:It requires that the destination device be configured to support Telnet connections.
Correct Answers:A, E
通过telnet程序发送的数据在网络中是以明文的方式（clear text format）传递的,而且如果远端设备需要对某台设备进行telnet访问，则必须在该设备上配置允许telnet访问。
In order to allow the establishment of a Telnet session with a router, which set of commands must be configured?
A: router(config)# line console 0
router(config-line)# enable passwordcisco
B: router(config)# line console 0
router(config-line)# enable secretcisco
router(config-line)# login
C: router(config)# line console 0
router(config-line)# passwordcisco
router(config-line)# login
D: router(config)# line vty 0
router(config-line)# enable passwordcisco
E: router(config)# line vty 0
router(config-line)# enable secretcisco
router(config-line)# login
F: router(config)# line vty 0
router(config-line)# passwordcisco
router(config-line)# login
Correct Answers:F
telnet是一个应用层的应用，他使用的是vty线路，而且在默认的情况下，是需要访问的线路下设有密码的。而在VTY线路下设置密码的命令为passworkstring,而VTY线路下的另一个命令login则是默认的，可写也可不写。如果想Telnet时在VTY线路下不设置密码也可以访问这个线路，可以在该VTY线路下输入命令no login。
What is the purpose of using the traceroute command?
A: to map all the devices on a network
B: to display the current TCP/IP configuration values
C: to see how a device MAC address is mapped to its IP address
D: to see the path a packet will take when traveling to a specified destination
E: to display the MTU values for each router in a specified network path from a source to a destination
Correct Answers:D
通过Traceroute命令我们可以知道信息从计算机到互联网另一端的主机是走的什么路径。它的用法和ping命令相似，只是ping用来测试连通性。
Refer to the exhibit. The DHCP settings have recently been changed on the DHCP server and the client is no longer able to reach network resources. What should be done to correct this situation?
A: Verify that the DNS server address is correct in the DHCP pool.
B: Ping the default gateway to populate the ARP cache.
C: Use the tracert command on the DHCP client to first determine where the problem is located.
D: Clear all DHCP leases on the router to prevent address conflicts.
E: Issue the ipconfig command with the /release and /renew options in a command window.
Correct Answers:E
DHCP服务器发生变化，client需要清除租约已过期的IP地址（ipconfig/release）然后从DHCP服务器上重新获取IP地址(ipconfig/renew).