Well, the only thing I still can propose is
1) Change all host names with IPs - maybe host name is resolvd wrong
2) Try setting TRACE_LEVEL_CLIENT=16 in SQLNET.ORA. This is going to dump the entire network packages into the trace file and hopefully will help
3) Try connecting to the server, using easy connect ( I assume you use 10g)
That means instead of connect string specify host: port/service
4) Put here the output of lsnrctl status and the fragment of the tnsnames.ora you use
If the host name or IP is incorrect, then the error would be TNS couldnt resolve service name. Thats fine and the client process is not finding any server process listening in the mentioned port. I suggest to try connect using sqlplusw and paste the error along with the error no in the forum. Pls ensure that u check the port number in the correct file.
I would disagree here! "TNS couldnt resolve service name" is an error, appearing when the string you entered in the 'connect string' field cannot be found in the tnsnames.ora (or to be precise - cannot be resolved to hostort:service_name) In case if tnsnames.ora contains such string and the string is resolved to HOST_NAMEort:sid, however DNS maps the host name to wrong IP address, where no database is installed at all, the error will be "No Listener" - I am pretty sure about that.
Anyway, the poster seems lost interes in that thread :-)
Thank you very much for the reply..but the problem was different.
The problem was I have the Check Point firewall in my system and it was blocking, but somehow other applications could overrode it. When I tried shutting down the firewall, it still didnt work, and I found out there is one more service is running at the background preventing it.
So finally I reinstalled the firewall and now I am able to connect.