When your router is connected to the Internet via your ISP, it is usually allocated a dynamic public IP address. The public IP address assigned to your router changes each time you reconnect to the ISP. This can cause a problem if you host a web server, FTP server, or other server behind the router. Dynamic DNS provides a solution for this.
Dynamic DNS (DDNS) is a method or network service that provides the capability for your Vigor router to notify a domain name server to change in real time the active DNS configuration of the configured hostname. This means that you can invite your friends or visitors to access your web server by using the domain name you have setup without having to worry about the actual IP public IP address of your Vigor Router.
There are various free services available on the Internet providing a Dynamic DNS service. Once registered, you have a name such as DrayTek.dyndns.org. It is mapped to your current IP address. Your Vigor Router is preconfigured with a list of these service providers.
For details on how to configure your Vigor router for DDNS Click Here