Destination Host Unreachable
This message indicates one of two problems: either the local system has no route to the desired destination, or a remote router reports that it has no route to the destination.
If the message is simply “Destination Host Unreachable,” then there is no route from the local system, and the packets to be sent were never put on the wire.
If the message is “Reply From < IP address >: Destination Host Unreachable,” then the routing problem occurred at a remote router, whose address is indicated by the “< IP address >” field.
Request Timed Out
This message indicates that no Echo Reply messages were received within the default time of 1 second. This can be due to many different causes; the most common include network congestion, failure of the ARP request, packet filtering, routing error, or a silent discard.
For more info Refer: http://technet.microsoft.com/en-us/library/cc940095.aspx