The host you are trying to connect to cannot be reached, either due to network problems or due to an incorrect setup of the client’s network.
So make sure you can reach the network, check your network card, drivers, …
Also, if you have a firewall in place, make sure it is not blocking access to the server/client.
You might also want to double check the host name entered, maybe there is a typo in it.
In following example we made an error in the ip address. Instead of a dot, we’ve put a slash
C:\>sqlplus user/pw@//192/168.1.20/ORCL SQL*Plus: Release 126.96.36.199.0 Production Copyright (c) 1982, 2010, Oracle. All rights reserved. ERROR: ORA-12543: TNS:destination host unreachable Enter user-name: C:\>sqlplus user/pw@//192.168.1.20/ORCL SQL*Plus: Release 188.8.131.52.0 Production Copyright (c) 1982, 2010, Oracle. All rights reserved. Connected to: Oracle Database 11g Enterprise Edition Release 184.108.40.206.0 - 64bit Production With the Partitioning, OLAP, Data Mining and Real Application Testing options