Why has my IP address changed?
Using Dialup Internet
If you are dialing in to your Internet Service Provider each time with a Telephone based modem each time you connect to the internet, you will be assigned an IP address that is available each time you connect to the internet. Because of the type of connection where you are only connected when you dial in, it would be almost impossible to connect with the same IP address every time. Therefore you are assigned a random IP on the network when you connect. If you are looking for a static (never changing) IP address, you must subscribe to an always-on internet connection (cable, dsl, t1, t3, etc…).
Using Broadband Internet
Generally, standard priced packages come with what is called a leased IP address. Not lease in the sense of the term that it costs you extra money, but where you are simply borrowing an IP address from the DHCP pool of your ISP for a defined period of time. Upon expiration of your lease, you are automatically assigned a new IP address and start a new lease.
If you are looking for a Static IP address, most ISP’s provide static IPs in premium or business grade packages. Call your ISP for more information.
If the reason you are looking for a static IP address is to self host your own website, e-mail, etc… you will be much better off instead leasing a VPS from a data center. Your bandwidth and reliability are much better and you will be able to retain your own high speed access to the internet. We used Wiredtree for our servers, they are amazing, check them out before self hosting your own website off your home broadband connection.