If you want to spare yourself from a flood of UNREACHABLE notifications during network outages, you can exclude the unreachable (u) option from the notification_options directive in your host definitions and/or the host_notification_options directive in your contact definitions.
https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/3/en/networkreachability.html
http://nagios-jp.osdn.jp/nagios-doc-2x-ja/networkreachability.html
Tip: Do not confuse host dependencies with parent/child host relationships. You should be using parent/child host relationships (defined with the parents directive in host definitions) for most cases, rather than host dependencies. A description of how parent/child host relationships work can be found in the documentation on network reachability.
https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/3/en/dependencies.html
