ORCL = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = server_a)(PORT = 1521)) (CONNECT_DATA = (SERVICE_NAME = orcl) ) )The most likely cause of this error is that the HOST name is incorrect or misspelled. If the HOST name is correct, confirm that this host exists by typing 'nslookup hostname' at the operating system command prompt. If the host does exist and your attempt to connect still fails, add the HOST name and IP address into your local hosts file and attempt to connect again. If this still fails, try replacing the HOST name in your tnsnames.ora file with the server's IP address and attempt to connect again.
If these suggestions do not resolve your problem, you should turn on Oracle Net tracing to gather further information...
that may assist in troubleshooting. The Oracle9i Release 2 (9.2) Net Services Administrator's Guide includes a section on troubleshooting Oracle net services.
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