ARP: Stands for Address Resolution Protocol…whenever a request is sent by a node on one network to the node on another network the Physical address(MAC) is required and for this the IP address need to be flow over the network..whenver a router with that network (IP) gets the msg. the required MAC address is sent through the network this process of converting the IP address to MAC address is Called ARP..and the reverse thats the convertion of the Mac address to the IP address is called RARP ( Reverse Address Resolution Protocol)