DDNS or Dydns is known as Dynamic DNS. It is basically a service which is offered for mapping domain names with a dynamically changing IP address. In the below topology, the router has been assigned a public IP address by the ISP.
Public IP addresses are assigned by the ISP for each connection. Without a public IP address, internet access would not be possible. ISP offers dynamic and static IP addresses. Dynamic IP addresses are not constant. The ISP would change it on a periodic basis. Static IP address is constant. They do not change. Take a scenario, where a authorized used is required to access the router from a different location. For this purpose, the user would connect to the public IP address of the router. If the IP address is static, there would not be any issues as the IP is constant. But if the IP address is dynamic and changes, then it would create a problem, as the IP address need to be tracked constantly for connectivity. For this purpose, the user can use a DYDNS service provider and register a domain name, following which a username and password is assigned. The username and password which is provided is configured on the router, which would then periodically update the service provider with the changing IP address. The user would not need to keep track of the changing IP addresses, but can access the router using the DNS name.