When you dial up(using a modem) to access the internet, your internet provider assigns you a dynamic IP address. Hence, this is not a fixed IP of yours, and when you disconnect from the service, and dial up again, the IP will be different. Many DSL connections also use this method. The advantage of this is, that you usually don't have to worry about someone trying to get into your computer, since you wouldn't be on the same IP for a long perid of time. The disadvantage is that you really can't run a DNS web server from your computer.