NTP pool
The NTP pool is a dynamic collection of networked computers that volunteer to provide highly accurate time via the Network Time Protocol to clients worldwide. The machines that are "in the pool" are part of the pool.ntp.org domain as well as of several subdomains divided by geographical zone and are distributed to NTP clients via round-robin DNS. The geographic zone selection is sometimes unnecessary, obviated either by authoritative DNS servers that use geolocation or anycast server addresses.