The router inspecters the packet’s IP address and performs  (1)   lookup to see if the destination end station resides on the local(physically connected)network, typically called an IP subber.  (2)   usually is assignted to each of the router’s network interface. If the destination IP address is local, the router searches an internal store of IP address and local-device Media Acess Control(MAC) address. This store is known as  the  (3)   cache. Address Resolution rotocol(ARP)is the universal tool for matching IP addresses to MAC address. If the destination’s MAC address appears, the router installs that MAC address in the pachet header(removing its own MAC address because that’s no longer needed) and sends the packet to the destional end station.
   In the event that the destional MAC address does not appear in the ARP cache-it might have timed out, for instance-the router must broadcast an ARP request to the subnet referenced by the packet’s destional IP address. The end station with that IP address responds, sending back its MAC address. The router updates its cache, installs the new MAC address into the  (4)   and launches the packet. If the router table lookup shows that the packet is destined for a non-local subnet, the router forwards the packet to the next-hop  router using the-hop router’s MAC address.Routing tables are continuously built and rebuilt by intelligent discovery protocols, such as Routing Information Protocol or Open Shortest Path First(OSPF).Each router’s routing table shows the best route to the destination address: for address that may be severd hops away, it shows  (5)   next-hop router.
 
(1) A. a router table      B.a signal    C.an e-mail        D.a file
(2) A. A TCP  subnet    B.IP header   C. An IP subnet    D.TCP/IP
(3) A.OSPF     B. Address Resolution Protocol(ARP)     C.MAC      D.RIP
(4) A.small packet      B.space      C.packet header     D.packet tailor
(5) A.the fewest        B.the worst   C.the least          D.the best #p#
答案:1、A  2、C  3、B  4、C  5、D

参考译文：

路由器检测包的IP地址，并执行一个路由器查表，看看目的地端站驻留在本地（物理连接）网络，通常称为IP subber 。通常一个IP子网是被定义的每个路由器的网络接口。如果目的IP地址是本地，路由器搜索内部存储的IP地址和本地设备的媒体访问控制（ MAC ）地址。这家店面被称为地址解析协议（ ARP ）高速缓存。地址解析协议（ ARP协议）是普遍的工具匹配IP地址的MAC地址。如果目的地的MAC地址时，路由器的安装MAC地址在包标题（除去自己的MAC地址，因为这不再需要）和发送数据包的目的端站。
   如果目的MAC地址没有出现在ARP高速缓存，它有可能超时，例如，路由器必须播出一个ARP请求的子网中所引用的数据包的目的 IP地址。年底站与IP地址的回应，发送回其MAC地址。路由器更新其缓存，安装新的MAC地址的数据包标头和发射的数据包。如果路由器查表表明，包注定是一个非本地子网，路由器转发数据包到下一跳路由器使用一跳路由器的MAC 地址。路由表不断兴建和改建的智能发现协议，如路由信息协议或开放式最短路径优先（ OSPF ） 。每台路由器的路由表显示的最佳途径到目的地地址：地址可能自动选择，它表明的最佳下一跳路由器。