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.
Bookmark this page and SHARE: