dvdnav:

[1] VLC command-line help - VideoLAN Wiki (2017-10-10 19:17:15 +09:00) <https://wiki.videolan.org/VLC_command-line_help/>

dvd://[device] DVD device

[2] MPlayer - The Movie Player (2018-04-18 05:15:11 +09:00) <http://www.mplayerhq.hu/DOCS/HTML/en/MPlayer.html#dvd>

The syntax to play a standard DVD is as follows:
mplayer dvd://<track> [-dvd-device <device>]
Example:
mplayer dvd://1 -dvd-device /dev/hdc
If you have compiled MPlayer with dvdnav support, the syntax is the same, except that you need to use dvdnav:// instead of dvd://.