第8套
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  Protocol(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 headerC. 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 
答案:1:A,2:C,3:B,4:C,5:D