IP address duplication detection method using address resolution protocol

An IP address duplication detection method using an address resolution protocol is used in a network system in which an IP address duplication diagnosis/detection host and a plurality of check target hosts are connected to a single LAN, all the hosts are respectively assigned MAC addresses as hardware addresses unique to devices without any duplication, and an address resolution protocol aimed at acquiring a MAC address of a remote device from an IP address is installed in each host. In this method, an ARP request packet containing a check target IP address in the same subnet as for the IP address duplication diagnosis/detection host is sent out from the IP address duplication diagnosis/detection host to each of the check target hosts through the LAN. IP address duplication is detected depending on whether an ARP reply packet as a response packet for the ARP request packet is returned from each of the check target hosts to the IP address duplication diagnosis/detection host through the LAN.