"Reachable" is just what it says; "Can this device (like a management platform)see and communicate with the target device.
PING is a basic, primary tool for reachability. Unfortunately, many virus/worms/trojans propagate by executing a "Ping Sweep" to find other devices to attack, so many public resources are set to not respond to Ping.
Some organizations expand this to filter Pings inside their network, to reduce the propagation of virus & worms in the event that somene carries one in.
Usually management platforms will poll for SNMP information for devices it knows about and understands. SNMP polls provide better information than "I can see it, and it responds."
For devices that a management platform doesn't know or understand (some other vendor's product, for example), Ping is frequently the only option to determine that the device is still active, available, and responsive ("reachable").
If Ping is filtered, some other method must be used (does it respond to Telnet? FTP?)
Hope this helps
Good Luck
Scott
