working principles
A NTP primary server, also called of stratum 1, is a computer connected to an high precision reference clock and equipped with a NTP software. Other computers, called of stratum 2, equipped with a similar software, automatically query the primary server to synchronize their system clocks. The stratum 2 computers can synchronize other computers, called of stratum 3, and so on until 16 stratums. Going away from stratum 1 toward lower stratums, the synchronization accuracy lowers. In such a framework, each computer can be at the same time a server for the computers belonging to the lower stratum and a client for the computers belonging to the upper stratum. Each server can have some hundreds of clients, so the number of computers that can be indirectly synchronized by one primary server is virtually unlimited. To make the system more reliable, each client can have more the one server in the upper stratum. In this case, the NTP software monitors continuously the figures of stability and accuracy of all configured servers, switching dynamically to the server with the best figures.

synchronization schemes
The figure shows same typical situations of NTP synchronization.

In the case BX a single computer is synchronized to the A1 stratum 1 server.
In the case BY , to have more synchronization reliability, the computer BY is configured with two synchronization servers A1 and A2.
In the case LANZ, an organization that want to synchronize the computers of its network, has chosen a classic solution: two computers of the local network have been set up as NTP local servers and synchronized to three NTP primary servers (A1, A2, A3), then all remaining computers (C1, C2, ..., Cn) have been synchronized to the local servers (B1, B2). This configuration reduces the traffic across Internet between the local network and the primary servers and keeps the configuration of the local network depending only on the local servers (B1, B2) that can be easily reconfigured against the external servers when it is needed.

what is needed
To implement a NTP client or a non-primary server, it is sufficient to install and to configure a NTP software. To implement a NTP primary server, among the software, an external reference clock it is needed, many choices are possible depending on needs and possibilities.

Software for NTP

There are two kind of NTP software: complete and simplified (SNTP). The complete kind implements all the features of NTP. The simplified kind implements a subset of NTP, it can only work as client and can only have defined 1 server at a time.