DUN (Dial Up Network File)
The DUN Settings file format
The file extension DUN has been used in former versions of Microsoft Windows, but you might still encounter them on old (external) hard drives. These files were used by Dial-Up Networking tools to store information about internet connection settings. For quick access of the DUN, a file with the appropriate extension was often stored on the desktop of a user's computer.
Technical details of DUN files
DUN files are not being used nowadays as Dial-Up Networking tools have been replaced by other way to connect a computer to the internet. The files store information the modem needs to successfully connect to the internet. This includes the phone number and other modem settings.