NoMachine Enterprise software can run in both environments. There are two types of license format available depending on your requirements.

When you purchase new subscriptions from the store on the NoMachine website, by default you receive what we call an "online license". The license file is installed once on the server machine which is then validated through the NoMachine licensing server. Internet access is therefore a requirement (through outbound access on HTTPS port 443) to allow the NoMachine server to periodically verify the validity of its subscription. You can read about this in more detail in the following articles: 

How online server subscription checks work https://kb.nomachine.com/AR11U01209

How do NoMachine server licenses work in v9 https://kb.nomachine.com/AR10U01206

If you need to activate NoMachine software which is running on hosts within an air-gapped network, where external internet access is blocked or severely restricted, you require an offline or on-premise license. We can provide you with the license file that does not require access to the Internet. Please contact our sales team with the details of your set-up and license requirements.