Periodically test the reachability state of your devices.
Perform periodic polling of your devices for reachability, either using ICMP ECHO, or, where the device is on a separate routable segment, using a simple connection to a listening echo port on the device.
Generate alerts when devices become unreachable, and automatically clear the alerts when the device become reachable again.
Create management reports detailing percentage uptime, based on your reachability polling.