This is a pretty useful feature and whilst you can use a local DAC connection by default. That is, when you are connected locally to the server in question, you can connect using the DAC. If you want to connect remotely you need enable the ‘remote admin connections’ server options.
You can check is the ‘remote admin connections’ is enabled by running the following query
where name like 'remote admin connections'
A value_in_use of 0 indicates local connections. A value of 1 means that remote connections are enabled
You can change the option if you need by using SP_CONFIGURE and the code below
sp_configure 'remote admin connections', 1
Enabling the DAC can be a lifesaver if you get a rogue query chewing up all the resources on a server and it can be a good idea to enable it. I wrote a post over 6 years ago saying to connect to the DAC you can see the link to that below
Connecting using the DAC - http://www.gethynellis.com/2010/09/sql-server-dedicated-administrator.html