This read-only IpAddress records the last invalid IP address used by the ComPort, which per the description is cleared every quarter hour. An admin checks this to see what unauthorized address a subscriber's device most recently attempted, useful for diagnosing a misconfigured or compromised device. For example, an admin investigating repeated ARP filter drops for a device inspects hcxStuIpArpFiltInvalidAddr to see exactly what unauthorized address it was trying to use.