/oracle/C12/network/admin/tnsnames.ora
If C12 has Release 4.x or higher with Oracle 8.1.x or higher: /oracle/C12/_/network/admin/tnsnames.ora
Make sure that the file there belongs to the user orac12 and has at least permissions 0644.
3. Changes in sqlnet.ora
No changes are necessary in sqlnet.ora.
4. Restart the Listener
After editing the files, restart the listener to make the changes effective.