14.2. An Overview of the TNS Listener
The Oracle TNS listener is a server process that provides network connectivity to the Oracle database. The listener is configured to listen for connection requests on a specified port on the database server. When an incoming connection request is received on the port, the listener will attempt to resolve the request and forward the connection information to the appropriate database instance.
A connection request will provide a username, a password, and an Oracle SID or service name. If the listener has been configured to listen for that particular SID or service name, it will forward the login information to that instance. If not, it will return an error message to the requester.
Once the database authenticates ...