You were just given a used Allen-Bradley Compactlogic PLC and no one has a clue what is in the controller….. Not even the IP address of the PLC. Without IP address, there is no way in communicating with the PLC… ok, ok you might say USB. Fine that is true but what if the PLC does not have USB? How do you find the IP address of the PLC. One network acronym… ARP. Also known as Address Resolution Protocol. Network devices “shoots” out an ARP request when a device cannot be found, its like a mother sheep calling its lamb when it cannot be found…. analogy is kinda bad but I think you get the point.