Port 1521 is the default port used for SQLnet, which is a protocol for establishing network sessions and data exchange between a client and an Oracle Database server. Different ports have their assigned services; for instance, port 1433 is commonly used by Microsoft SQL Server, while port 3306 is the default port for MySQL database servers. Port 22 is typically used for SSH (Secure Shell) connections, which is useful for secure logins, file transfers, and TCP port forwarding.