TNS (Transparent Network Substrate) names are contained in a file called tnsnames.ora, that defines outgoing database requests.
It contains all database names (SIDs) running on the processor.
When a new database is added to a box, the file /etc/tnsnames.ora must be updated otherwise you won't be able to connect to the database in PL/SQL Developer.
This file also describes each domain name, including protocol, host, port and service name information .
You can find TNS Names in PL/SQL Devloper by navigating to the 'Help' menu in menu bar and under that; 'Support Info'. Once you click on 'Support Info' a small window will appear containing six tabs; and in these tabs you will find one with the name "TNS Names".
