172.16.1.5为什么不能访问172.16.1.3？ 
正确答案:
PCA in networkA will directlly request for PCB’s mac address using ARP without request for the default gateway’s mac address because it considers that PCB is in the same subnet. The router doesn’t forward the broadcast ARP request message. ARP process failure and PCA cannot get the MAC address of PCB ，so PCs in the two network cannot communicate with each other.
答案解析:有
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