Installing multiple default instances on a single server
As you probably know, SQL Server allows only one default instance per server. The reason is not actually something special to SQL Server, but it has to do with the way TCP/IP endpoints work.
In fact, a SQL Server default instance is nothing special compared to a named instance: it has a specific instance id (MSSQLSERVER) and listens on a well-known TCP port (1433), but it has no other intrinsic property or feature that makes it different from any other instance.
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