Ans:
DDNS stands for Dynamic DNS, is a service for transforming dynamic IP to corresponding to a specific "Hostname". There are many websites providing free DDNS services. For example, http://www.dyndns.org.

Cited from Wikipedia: Dynamic DNS is a system for allowing an Internet domain name to be assigned to a computer with a varying IP address. This makes it possible for other sites on the Internet to establish connections to the machine without needing to track the IP address itself.

Dynamic DNS service is provided on a large scale by various DNS hosting services, which retain the current addresses in a database and provide a "client" program to the user who will send an update to the service whenever the server's IP address has changed. Many routers and other networking components contain a feature such as this in their firmware.