Have you ever wondered why some SQL Server instances are referred to as “ServerName/InstanceName:PortNumber”? You may have thought that using both the InstanceName and PortNumber is redundant, and that it’s best practice to use just the InstanceName. However, there are cases where using the PortNumber is actually necessary.

Let’s start with some background information. By default, the default instance of SQL Server uses port 1433. Named instances, on the other hand, use a dynamic port that is generated the first time the instance is started. This dynamic port can change each time the instance is restarted, although it’s rare.

Now, you might be thinking, why not just use the InstanceName instead of the PortNumber? After all, the instance name is typically used to connect to the SQL Server. Well, here’s where the browser service comes into play. When you use the instance name, the SQL Server first asks the browser service (using port 1434) for the port number associated with the named instance. This two-step process ensures that the connection is made using the correct port number.

So, when should you use the PortNumber instead of the InstanceName? The answer is when there is a firewall involved. Firewalls require specific ports to be opened, and if the port changes dynamically, the new port may be blocked by the firewall. This can be a problem, especially in production environments. Therefore, it’s advisable to set the port to a static value if you are working behind a firewall.

Additionally, using the PortNumber can help avoid the need to open port 1434 in the firewall. When you use the InstanceName, two ports need to be opened in the firewall – the port for the SQL Server instance and port 1434 for the browser service. However, port 1434 is well-known and may pose a security risk.

In some cases, using both the InstanceName and PortNumber can be beneficial. For example, if you have a complex server naming convention, including the instance name makes it easier to understand which instance you are referring to. It’s also worth noting that using a name is more human-friendly than using a number.

In conclusion, while it’s generally recommended to use the InstanceName to connect to SQL Server instances, there are scenarios where using the PortNumber is necessary, particularly when working behind a firewall. Understanding the role of the browser service and the implications of dynamic ports can help you make informed decisions when configuring your SQL Server instances.

Thank you for reading!