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.
Work is being done to make the geographic zone selection unnecessary via customized authoritative DNS servers that utilize geolocation software.
[4][5][6] The more time servers there are in the pool, the lower the resource demand on each member.
Joining the pool requires at least a broadband connection to the Internet, a static IP address, and accurate time from another source (for example, another NTP server, a DCF77 receiver, a WWVB receiver, or a GPS disciplined oscillator).
This project was started by Adrian von Bidder in January 2003 after a discussion on comp.protocols.time.ntp about abuse of the public stratum 1 servers.