The Penguin's Practical Network Troubleshooting Guide - page 3
Start with Cable Testing
Destination Host Unreachablemeans that
pinggot as far as a router that is local to the remote host, but it cannot reach the remote host.
unknown host means you entered the wrong hostname, DNS is broken, or you are not connected to the network. Ping the IP to see if it's a DNS or connectivity problem.
If you are on a multi-homed box, use
ping's -I flag to select a single interface:
# ping -I eth1 [IP or hostname]
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