I came across an interesting situation where a developer was trying to connect to a named instance using a DNS alias without specifying the instance name. Coincidently though he remembered to include the port number and miraculously it worked. So it appears that sql server accepts connections to a specific instance based on its port number.

While it may not seem to particularly useful, I can imagine it could be used in the following situations:

• To mirror to a server with a different instance name (but same port number)
• To hide the complexity of instance names from end users and just rely on port number (and optionally dns alias)