Which port is used by the Oracle Database listener?

• A. 636 TCP
• B. 3389 TCP
• C. 1883 TCP
• D. 1521 TCP

Answer: (D)

The Oracle Database listener is the component that accepts incoming client connections and routes them to the correct database service. It listens on a TCP port, and by default that port is 1521. This default is defined in Oracle Net configuration and is the standard port clients use to reach the listener when connecting with TNS descriptors. When a client connects to 1521, the listener allocates the session and hands it off to the appropriate database instance. The other ports listed correspond to different services (for example, 636 is used for LDAP over SSL, 3389 for Remote Desktop, and 1883 for MQTT), so they aren’t used for the Oracle listener. You can change the listener port if needed, but 1521 remains the conventional default.