The NTP system is based on Internet time servers, computers with access to atomic clocks such as those operated by the U.S. government. These NTP servers run a software service that provides the clock's time of day to client computers over UDP port 123. NTP supports a hierarchy of multiple server levels to handle a large load of client requests. The protocol includes algorithms to accurately adjust the time of day being reported to account for Internet network transmission delays.
Computers running Windows, Mac OS X and Linux operating systems can be configured to use an NTP server. Starting with Windows XP, for example, the Control Panel "Date and Time" option contains an Internet Time tab which allows choosing an NTP server and turning time synchronization on or off.