Error 1: Connect failed because target host or object does not exist
Solution : goto run->drivers->etc->hosts file and open it with notepad. Then do add a entry.
Note : If it is unix machine, check in /etc/hosts.
Error 2 : protocol adapter error
Solution : Generally it will occur if network connectivity is not there. So ping the serverand if its working fine then check if firewall is enabled either at client side or server side.
Error 3 : connection description doesn’t found
Solution : check tns entry in tnsnames.ora for any syntax error.
Note : port number in listener.ora and tnsnames.ora should be same.