This is an odd one that I've never seen before. I'm hoping someone can shed some light.
I have a SQL Server database with a default instance and a named instance. For the sake of this conversation we'll call the named instance SQL2.
Using the local IP address I can connect to the named instance just fine. See screenshot below.
However, using the public IP address the connection fails. See similar screenshot below. (Public IP address is made up)
Further complicating things is that if I use the explicit port number instead of the named instance it works again.
I've never seen a case of a named instance failing to connect like this. There must be some kind of configuration issue I'm overlooking. Can anyone suggest a solution?