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Oracle DB Listener
Port 1521 is the default network listener port used by Oracle Database systems for client-server communications. The listener process manages incoming client connections, routing them to the appropriate database services. It plays a critical role in database connectivity, administration, and high-availability features for Oracle environments..
Port 1521 is traditionally used by the Oracle TNS (Transparent Network Substrate) Listener service, which is a fundamental component in Oracle database architecture. The listener acts as a gateway facilitating client connection requests by accepting these on this port and dispatching them to respective Oracle database instances. This facilitates multiplexed access, load balancing, and failover capabilities.
Communication over this port generally uses Oracle's proprietary SQL*Net or Net8 protocols, now commonly referred to as Oracle Net Services. These protocols encapsulate SQL commands and transaction data, enabling clients running Oracle client libraries or interfaces such as JDBC/ODBC or Oracle's own tools to interact with the database seamlessly.
Although the official IANA port for Oracle Net is now 2483, port 1521 remains the widely adopted default in existing deployments because of historical use and backward compatibility. Proper listener configuration is vital to ensure high availability, manageability, and performance of Oracle database services.