 Some modern operating systems, such as Apple iOS and Android, especially in mobile devices, are designed to randomize the assignment of a MAC address to network interface when scanning for wireless access points to avert tracking systems.In Internet Protocol (IP) networks, the MAC address of an interface corresponding to an IP address may be queried with the Address Resolution Protocol (ARP) for IPv4 and the Neighbor Discovery Protocol (NDP) for IPv6, relating OSI Layer 3 addresses to Layer 2 addresses.According to Edward Snowden, the US National Security Agency has a system that tracks the movements of mobile devices in a city by monitoring MAC addresses.