NIST Internet Time Servers

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The table below lists the time servers used by the NIST Internet Time Service (ITS). The table lists each server's name, IP address, and location, organized geographically within the US from North to South and then from East to West. Please note that while we make every effort to ensure that the names of the servers are correct, we control the names of only the nist.gov servers. If you have difficulty using the name of a system, you can access a server using the IP address directly.

Important notes:

 1. We will continue to support the "TIME" protocol that uses tcp port 37 for the forseeable future. However, this protocol is very expensive in terms of network bandwidth, since it uses the complete tcp machinery to transmit only 32 bits of data. Users are *strongly* encouraged to upgrade to the network time protocol (NTP), which is both more accurate and more robust.

2. Users of the NIST "DAYTIME" protocol on tcp port 13 are also strongly encouraged to upgrade to the network time protocol, which provides greater accuracy and requires less network bandwidth. The NIST time client (nistime-32bit.exe) supports both protocols.

3. The generic name time.nist.gov will continue to point to all of our servers on a round-robin basis, and users are encouraged to access the service using this name.


The global address time.nist.gov is resolved to all of the server addresses below in a round-robin sequence to equalize the load across all of the servers.

Whether you connect to a server using the name or the IP address, it is a bad practice to “hard-code” a particular server name or address into a device so that these parameters cannot be changed bu the end user if that becomes necessary at some future time.

All users should ensure that their software NEVER queries a server more frequently than once every 4 seconds. Systems that exceed this rate will be refused service. In extreme cases, systems that exceed this limit may be considered as attempting a denial-of-service-attack.

Name IP Address Location Status
time-a-g.nist.gov 129.6.15.28 NIST, Maryland All Services Available
time-b-g.nist.gov 129.6.15.29 NIST, Maryland All Services Available
time-c-g.nist.gov 129.6.15.30 NIST, Maryland
time-d-g.nist.gov 129.6.15.27 NIST, Maryland All Services Available
time-d-g.nist.gov 2610:20:6f15:15::27 NIST, Maryland All Services Available
time-e-g.nist.gov 129.6.15.26 NIST, Maryland All Services Available
time-e-g.nist.gov 2610:20:6f15:15::26 NIST, Maryland All Services Available
time-a-www.nist.gov 132.163.97.1 WWV, Colorado All Services Available
time-b-www.nist.gov 132.163.97.2 WWV, Colorado All Services Available
time-c-www.nist.gov 132.163.97.3 WWV, Colorado All Services Available
time-d-www.nist.gov 132.163.97.4 WWV, Colorado All Services Available
time-d-www.nist.gov 2610:20:6f97:97::4 WWV, Colorado All Services via IPv6
time-e-www.nist.gov 132.163.97.6 WWV, Colorado All Services Available
time-e-www.nist.gov 2610:20:6f97:97::6 WWV, Colorado new server,services via IPV6
time-a-b.nist.gov 132.163.96.1 NIST, Colorado All Services Available
time-b-b.nist.gov 132.163.96.2 NIST, Colorado All Services Available
time-c-b.nist.gov 132.163.96.3 NIST, Colorado All Services Available
time-d-b.nist.gov 132.163.96.4 NIST, Colorado All Services Available
time-d-b.nist.gov 2610:20:6f96:96::4 NIST, Colorado All Services Available
time-e-b.nist.gov 132.163.96.6 NIST, Colorado All Services Available
time-e-b.nist.gov 2610:20:6f96:96::6 NIST, Colorado All Services Available
time.nist.gov global address for all servers Multiple locations All Services Available
utcnist.colorado.edu 128.138.140.44 University of Colorado,Boulder All Services Available
utcnist2.colorado.edu 128.138.141.172 University of Colorado,Boulder All Services Available
utcnist3.colorado.edu 128.138.140.211 University of Colorado,Boulder NTP only, Recommended

The following servers support only authenticated NTP requests using the symmetric key encryption method that is defined in the NTP documentation. They do not respond to requests for time in the DAYTIME or TIME formats and will not accept anonymous ftp connections. You must apply to NIST for an encryption key to use these systems; they will not respond to NTP requests from users who have not registered with NIST.

Name IP Address Location Status
ntp-b.nist.gov 132.163.96.5 NIST, Colorado Authenticated Service
ntp-www.nist.gov 132.163.97.5 NIST WWV, Colorado Authenticated Service
ntp-c.colorado.edu 128.138.141.177 JILA, Colorado Authenticated Service
ntp-d.nist.gov 129.6.15.32 NIST, Maryland Authenticated Service

The following server supports only the NTP format and transmits UT1 time rather than UTC(NIST).

ut1-time.colorado.edu 128.138.140.50 University of Colorado, Boulder
ut1-www.nist.gov 132.163.97.7 NIST WWV Radio, Ft. Collins, CO