DAC feature added from the SQL Server 2005 version. Using DAC feature a Database Administrator can connect to a SQL Server Instance when the database engine is not responding to regular connections and troubleshoot the issue.
Advantage of using DAC is DBA can connect to a SQL Server Instance and execute T-SQL commands to troubleshoot and fix issues rather than rebooting the SQL Server which could lead to database corruption or other problems.
By default DAC is disable, it is a best practice to enable the DAC.
We can enable the DAC using below T-SQL command.
Use master GO sp_configure 'show advanced options' , 1 GO sp_configure 'remote admin connections', 1 GO RECONFIGURE GO
In SQL Server 2008 to enable the DAC, right click on SQL Server and Select facets and from facets drop down Select Surface Area Configuration.