Veja o artigo Host de destino inacessível - Razão e solução .
Explica as possíveis razões do erro e dos conselhos:
- Make sure that local host is configured correctly
- Make sure Destination Computer/Device is up
- Disable the Firewall and check for the issue.
- Perform a tracert to the destination IP and check where the problem lies
Também fornece vários links úteis.