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

If a packet is to be forwarded to a host on a network that is directly connected to the router (i.e., the router is 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 Unreachable, Code 1 (Host Unreachable) ICMP message.

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).