A public NTP Server is a time server on the Internet that, as the name suggests, members of the public can use as a timing source. The best location on the Internet to find a list of public NTP servers is the home of NTP – www.ntp.org
There are two lists of public NTP servers on ntp.org, one for primary servers and one for secondary servers. Primary servers have up to several hundred clients each. However, many primary servers are ‘closed access’ meaning that only agreed clients can access them. This is because if there is too much traffic attempting to receive a timing source from a primary source then it will clog the network making the server useless.
Primary servers are known as a stratum 1 server in that they get their timing source direct from an atomic clock often using the GPS or national time and frequency transmissions. Secondary NTP servers tend to be stratum 2 time servers, that is a time server that receives its timing source from a stratum 1 server.
Most users that require a public NTP server will find that most primary servers are closed access and that they will have to use a secondary NTP server. When using a public NTP server it is important that access policies are adhered to as many institutions require on these servers for timing information.