Something that some companies like to do is to change the port number that the default instance is listening under as a security precaution. However this has a habit of stopping anyone from connecting to the default instance without knowing the port number.
This is because the default instance doesn’t register itself with the SQL Browser when it starts, so you can no longer simply just connect to the default instance when named instances will work just fine. The fix from CSS is to simply change the port back to TCP 1433. The reason for this is that changing the port number doesn’t do a lot to secure your SQL Server as a quick port scan will show an attacker which port the SQL Server is listening on. That or they’ll simply check out the web.config and get the port number from there.