图21.4使用ARP的四种情况 Target IP address: Target IP address: Destination address in the IP datagram IP address of a router Sender Sender Host Host Host Router LAN LAN × Receiver Receiver Case 1.A host has a packet to send to Case 2.A host wants to send a packet to another host on the same network. another host on another network. It must first be delivered to a router. Target IP address: Target IP address: IP address of the appropriate router found in the routing table Destination address in the IP datagram Sender Sender Host Router Router Router LAN LAN Receiver Receiver Case 3.A router receives a packet to be sent Case 4.A router receives a packet to be sent to a host on another network.It must first to a host on the same network. be delivered to the appropriate router. 21.7 21.7 图 21.4 使用ARP的四种情况