It's not enough to build hosts, systems and services - you also have so monitor them so that they stay up and available to your users. Nagios - formerly NetSaint - is a monitoring (and measuring) framework for all of the above. Both commercial (Nagios XI) and non-commercial (Nagios Core) versions are available for download but in this course we will cover Nagios Core and its configuration.
Learners without access to resources to spin up their own Nagios Core instance may use the online demo at https://exchange.nagios.org/directory/Demos/Nagios-Core-Online-Demo/details.