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Address resolution protocol (ARP) is widely used to maintain mapping between data link (e.g. MAC) and network (e.g. IP) layer addresses. Although most hosts rely on automated and dynamic management of ARP cache entries, current implementation is well-known to be vulnerable to spoofing or denial of service (DoS) attacks. There are many tools that exploit vulnerabilities of ARP protocols, and past proposals to address the weaknesses of the `original` ARP design have been unsatisfactory. Suggestions that ARP protocol definition be modified would cause serious and unacceptable compatibility problems. Other proposals require customised hardware be installed to monitor malicious ARP traffic, and many organisations cannot afford such cost. This study demonstrates that one can effectively eliminate most threats caused by the ARP vulnerabilities by installing anti-ARP spoofing agent (ASA), which intercepts unauthenticated exchange of ARP packets and blocks potentially insecure communications. The proposed approach requires neither modification of kernel ARP software nor installation of traffic monitors. Agent uses user datagram protocol (UDP) packets to enable networking among hosts in a transparent and secure manner. The authors implemented agent software on Windows XP and conducted an experiment. The results showed that ARP hacking tools could not penetrate hosts protected by ASA.