ICMP reason subcode 0x1, used with ICMP reason code 0x3 to explain why a DestinationUnreachable message was sent.

When a datagram is being forwarded, and reaches a router attached to the network a destination host is to be found on (i.e, the last hop router), and the router has ascertained that there is no path to the destination host then the router must generate a Destination Host Unreachable

It is usally generated by a gateway when it does not have a route for the destination specified in the IP packet that caused the error. Often this reason is used when a router cannot contact the next hop router specified in it's routing table (ie. The next hop router is not responding to ARP requests).