DDNS is a service that is used mostly for port forwarding so you can see the device on your network and from a WIFI network other than you own. For example, a Camera you may want to see both on your network and on some remote network while you are traveling. You would set up your ISP IPv4 address with a DDNS provider such as no-ip, setup an address reservation for your camera, and assign a port to the camera's IP address. Then you would setup the camera app by using settings something like xxxxx-ddns.com and port number. Now you can view the camera both on your network and on outside networks. If you used an internal IP of say 192.168.0.119, you could only see the camera within your network.