The mapping of Layer 3 (IP) to Layer 2 (MAC) addresses is a key service in IP networks, and is achieved via the ARP protocol in IPv4, and the NDP protocol in IPv6. Due to their stateless nature and lack of authentication, both ARP and NDP are vulnerable to spoofing attacks, which can enable Denial of Service (DoS) or man-in-the-middle (MITM) attacks. In this paper, we discuss the problem of ARP spoofing in the context of Software Defined Networks (SDNs), and present a new mitigation approach which leverages the centralised network control of SDN.