First of all, DNS has nothing to do with Mirrorcast.
The reason why it's less likely to have Mirrorcast on a Windows 7 laptop is that to use it requires minimum specifications, however, it is possible . I can cast from the laptop to my Samsung TV but not from my IPhone.
To do so on Windows 10 , go to settings/system/display, then scroll down to multiple displays and click connect to wireless display, to the right of the screen the options will appear, click into your TV option and look for the message on the TV so as to give permission.
I never use it much and it's a little hit and miss.