In the above topology, PC2 is shutdown. PC1 pings PC2. The first requirement in sending
out the packet from PC1 is to identify the mac-address of PC2, for which, PC1 sends an

ARP request. PC1 waits for an ARP reply. Since PC2 is shutdown, the ARP reply is not

send, and the ARP request times out. So the packet is never sent out from PC1. In a

second scenario, the ARP cache in PC1 contains the mac-address of PC2. In this case, the

packet (icmp) is constructed and sent out from PC1. Since PC2 is shutdown, the reply is

never sent and ultimately the packet times out.