ICMP Host Unreachable (U)

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When a datagram is being forwarded, and reaches a gateway attached to the network a destination host is to be found on, and the route to the host is down, the host is not responding or does not respond on the service port in question, a Host Unreachable message is sent back.

This message is usually displayed as a 'U' in a ping result, and as '!H' in a traceroute result.

 

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