Abstract: A method in a LAN test instrument for detecting proxy ARP agents and misconfigured routers in a TCP/IP LAN is provided. The method first allows for detection of routers running proxy ARP for the default route by issuing a single ARP command for a unique remote host. Because the target IP address chosen as a unique, non-existent device, the router will respond to the ARP request with a proxy ARP reply for its default route if this function is enabled. The method further allows for distinguishing between responses from actual devices having duplicate IP addresses and responses from misconfigured routers replying in proxy ARP for local hosts. The false duplicate IP addresses due to proxy ARP replies may be identified as phantom entries in a data base that can then be appropriately tagged or eliminated to display only the entries that correspond to actual physical devices to the user of the LAN test instrument.