SQL Server Connectivity – Setting a Static Port

Are you connecting to a named instance? In which case, the initial connection is to the SQL Server Browser service (UDP 1434) and then that dynamically allocates a port for named instances. This port is allocated on instance startup.

See this TechNet Resource on how the SQL Server Browser Service allocates a port for named instances.

There is a way to explicitly define a static port that a named instance will be listening on, but it appears that you are seeing a dynamically allocated port.

Please do not change Sql Server's TCP port unless you know what you are doing. This needs to be done only in specific scenarios like manual SPN registrations with failover cluster setups or restricted environments.

Sql Server installations contain a special service called Sql Browser Service. When a client tries to connect to a named instance, it will ask connection details from the browser service. As per the documentation:

Upon startup, SQL Server Browser starts and claims UDP port 1434. SQL Server Browser reads the registry, identifies all instances of SQL Server on the computer, and notes the ports and named pipes that they use --

When SQL Server clients request SQL Server resources, the client network library sends a UDP message to the server using port 1434. SQL Server Browser responds with the TCP/IP port or named pipe of the requested instance. The network library on the client application then completes the connection by sending a request to the server using the port or named pipe of the desired instance.