Internet Time Service Firewall information | NIST
https://www.nist.gov/pml/time-and-frequency-division/time-services/internet-time-service-firewall-information
WEBMay 10, 2010 · udp port 123, which is used by the network time protocol and the simple network time protocol. The NIST client software (and software from other sources) can be configured to use this port, but often does not use it by default. The NIST servers listen only on this port for NTP requests.
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