Most of the time ISPs provide u with a dynamic ip.A dynamic ip is one that changes ever so often(usually each time u switch on ur modem).A static ip on the other hand is one that remains constant and will not change.If u need a static ip u need to set it up yourself or ask ur ISP.If u want to try setting it up jus check the following Link.
www.portforward.com/networking/staticip.htmportforward.com/networking/staticip.htm