Internet Protocol address are basically two types public ip and private ip.In the layman point of view “A public IP address is assigned to every computer that connects to the internet, while private IP address are used to distinguish between computers on the same local area network(LAN).”
User has no control over the IP address (public) that is assigned to the computer. The public IP address is assigned to the computer by the Internet Service Provider as soon as the computer is connected to the Internet gateway.
A public IP address can be either static or dynamic. A static public IP address does not change and is used primarily for hosting web pages or services on the Internet. On the other hand, a dynamic public IP address is chosen from a pool of available addresses and changes each time one connects to the Internet.
Most of the internet user use dynamic IP. Thats why when the user reconnect to internet there IP address changes.While http://www.google.com have a static IP.
You can check your public IP address by visiting www.whatismyip.com