Your network (NIC) card comes from the manufacture with a hard wired address that it uses to communicate with the ISP. When your computer is first turned on, and when it connects to the ISP, the hardware and software automatically decide how they are are going to talk to each other, and what addresses they will accept as valid. The ISP assigns an Internet address to your computer (in most instances the address is different every time you log on). Unless you are only interested in setting up a private local network, you can not change the ISP assigned address or you will loose your internet connection.