Prior to SQL Server 7/2000 there was only the default instance of the server which automatically used the server’s name (NetBios). 7/2000 allow multiple instances allof which must be named something other than the server’s NetBios ID.
If you have turned off NetBios then you attach to the SQL server by IP or ‘named pipes’. If you are using NetBios and Active Directory to identify the instance to connect then you have to look at properties.
REMEMBER changing the instance name may require visiting every workstation to update the ODBC connector.
That is why I use the IP address and some port other than 1433.