What is NTP? Network Time Protocol Overview

Network Time Protocol (NTP) is an application layer protocol used for time synchronization between hosts on a TCP/IP network. Its goal is to ensure that all computers on a network agree on the system clock since even a small difference can create problems. For example, if there is more than 5 minutes difference between your host’s local clock and the Active Directory domain controller, you cannot log into your AD domain.

NTP uses a hierarchical system of time sources. At the top of the structure are highly accurate time sources – typically a GPS or atomic clock, providing Coordinated Universal Time (UTC). A reference clock is known as a stratum 0 server. Stratum 1 servers are directly linked to the reference clocks or stratum 0 servers and computers run NTP servers that deliver the time to stratum 2 servers, and so on (image source: Wikipedia):

what is ntp


NTP Server

NTP uses a client-server architecture; one host is configured as the NTP server and all other hosts on the network are configured as NTP clients. Consider the following example:

how ntp works

Host A is our NTP client and it is configured to use a public NTP server uk.pool.ntp.org. Host A will periodically send an NTP request to the NTP server. The server will provide accurate data and time, enabling system clock synchronization on Host A.


NTP servers listen for NTP packets on User Datagram Protocol (UDP) port 123. The current version is NTPv4, and it is backward compatible with NTPv3.

NTP servers can either be local or public. Public NTP servers are often free to use and are provided by third-party operators such as Google and Facebook. A local or internal NTP server is owned by the company itself and is deployed within the network.


Simple Network Time Protocol (SNTP)

Simple Network Time Protocol (SNTP) is a simpler NTP version commonly used by small networks. SNTP is a client-only architecture, so it can only receive information from the NTP time servers and cannot provide time for other devices.

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