No route to host.

socket operation was attempted to an unreachable host.

TCP/IP scenario: In BSD-compatible implementations, the local network system generates this error if there isn't a default route configured. Typically, though, Winsock generates WSAENETUNREACH when it receives a "host unreachable" ICMP message from a router instead of WSAEHOSTUNREACH. The ICMP message means that the router can't forward the IP datagram, possibly because it didn't get a response to the ARP request (which might mean the destination host is down).