(Dynamic DNS) A service that lets anyone on the Internet gain access to resources on a local network when the Internet address of that network is constantly changing. Such resources are typically a Web server, Webcam or a PC for remote control operation.

Cable and DSL providers frequently change the IP address of their customer's service, which makes it impossible to access the network from the outside world without knowing what the IP address is at any given moment. DDNS client software resides in a router, Webcam or PC. When it detects that the IP address of the cable or DSL modem has changed, it notifies the DDNS service provider of the new address. See dynamic IP address.

The DDNS Service Provider
The key element in this is the DDNS service provider, which hosts the DNS servers that become the authority for the domain names. When the DNS server responds to a request for the IP address of a domain name, it returns the current address along with a time-to-live (TTL) of only a few minutes. By causing the address to expire in such a short time, it prevents the address from being cached throughout the Internet. See TTL.

The DDNS provider generally offers a faster and more economical way to gain external access to internal resources than by upgrading ISP service from dynamic addresssing to a static address. The largest DDNS provider is Tzolkin Corporation (www.tzo.com), which was founded in 1997. See DDNS relay, DHCP, WINS and DNS.