This article applies to the online subscriptions, available since v9. The subscription file, server.lic, must be installed once on the NoMachine server host, and will be validated through the NoMachine licensing server.

Pre-requisite
Ensure that the host where the NoMachine Server is installed can establish outbound HTTPS connections (port 443) to license.nomachine.com.
This connection is required for the server to validate and maintain the subscription with the NoMachine licensing service.
In most environments, outbound HTTPS traffic is allowed by default. However, in corporate or restricted networks, it may be necessary to configure a firewall rule allowing outbound connections to https://license.nomachine.com on port 443.

Ordinary check, at server startup
Each time your NoMachine server is started or restarted (included for a reboot of the host machine), it checks the validity of the subscriptions against the NoMachine license server. If your NoMachine server cannot reach our license server upon start-up, it will try again at 48 hours. If it is still unable to establish a connection, it will stop working.

Ordinary periodic check
Your NoMachine server verifies the validity of its subscription every 48 hours. If your NoMachine server is not able to reach our license server, your server will continue to accept NoMachine connections for 48 hours until the next periodic check. If a connection to the NoMachine license server still cannot be established, next checks will be after 12h, 6h and 3h for a total of 96h interval of time. After that, the server will stop working. 

Moving of the license from a machine to another
The license is linked to the UUID of the NoMachine server the first time it is used. Uninstalling the server will remove the association between server.lic and server's UUID. It will be then possible to re-use the license file on a different machine. The association can be removed also by hand from command line (nxserver --subscriptionremove). Please consult the instructions for full details on how to migrate an online server or node license: https://kb.nomachine.com/AR10U01206

Online licenses on clone machines for backup purposes
An internal mechanism is implemented to exclude the possibility to use the same online license on a clone machine.