While Nagios Core is still very useful when configured to monitor only a simple list of hosts and services, it includes some optional directives that allow you to define some structural and functional properties of the monitored network—specifically, how hosts and services interrelate. Describing this structure in the configuration calls for some additional intelligent behavior in the monitoring and notification that Nagios Core performs.
Host parent definitions allow an administrator to define the hierarchy of connectivity to monitored hosts from the "point of view" of the Nagios Core server. An example might be a server with the monitored address in another subnet linked to the Nagios Core server by a router. If the router enters the
DOWNstate, this triggers Nagios Core's host reachability logic to automatically determine which hosts will become inaccessible and flags these as