What is Dynamic DNS?
DNS is a way to map human-readable domain names (such as cadzow.com.au) to IP addresses (such as 192.168.1.66). This is necessary because it is easier to remember the address of a website if expressed as words and not numbers. Normally DNS relies on the IP address not changing, or changing during predictable periods so that the records can be updated.
People who connect to the internet using a dial-up (modem) or retail broadband connections will receive a different IP address every time they connect. This is acceptable because, although every internet-connected device needs a unique IP address, the actual value does not matter. However, if you are hosting a server on your internet connection, such as a web server, or Terminal Services, if the IP address changes every time a connection is made, users will be unable to find the server reliably.
The usual solution is to obtain a fixed IP address from your Internet Service Provider. However, fixed IP addresses are relatively expensive (in the realm of $10-$20/month) so it is not always economic.
The solution is Dynamic DNS.
Dynamic DNS is a way to map human-readable domain names to IP addresses which may change frequently.
DynDNS.org and No-IP.com provide it as a free service, although there are other vendors of Dynamic DNS services.
The way it works is you obtain from the Dynamic DNS provider a domain name. The free Dynamic DNS service providers limit the domain names you can choose, so it might be of the form smith.dyndns.biz. (If you want your own domain name, it's best just to obtain one commercially.)
Next, you configure your router with the details of the domain name. The router then sends a message to the Dynamic DNS provider every time a new IP address is obtained, so they can update their systems with the new value.
Thus it is possible to reference the server using it's domain name of smith.dyndns.biz instead of trying to determine the IP address in case it has changed.
Note that although broadband/ADSL connections are “always on”, there are many circumstances where the connection will drop and the router/modem needs to reconnect. So an “always on” connection is no guarantee that the IP address will remain the same. It simply means there is not the same imperative to hang up when finished as with dial-up internet access.