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 |
