Owing to its great need for mapping an IP address to the corresponding MAC address over an Ethernet topology, Address Resolution Protocol (ARP) has been, and still is, capable of accomplishing this task efficiently. At the same time, it suffers from some security shortcomings, because of the malicious hosts have the possibility of poisoning the ARP cache for another host on the same LAN. In this paper, by gratuitous ARP request packets, we propose a solution to the problem of ARP poisoning. Our suggested mechanism which is named a Gratuitous Decision Packet System (GDPS) seeks to achieve two main goals: (1) Detection of suspicious ARP packets, by implementing a real-time analyzing for received ARP packets. (2) The distinction between a legitimate and malicious host through sending a modified request packet of the gratuitous ARP packets. Furthermore, the experiments show that the presented design has the efficiency and accuracy, as well as it does not require any additional software or hardware.