Thanks for your question kittybanks.
That's an awfully vague question.
Many ISP's change or rotate IP's. They say it's for the subscribers security but the truth is that there is no added security in changing your IP address. They really do it to try to force compliance with the user agreement that typically says subscribers are not supposed to host Internet servers on their residential connections. That would usually include email servers, web servers and game servers. The point of all that is to cut down on spam servers. It also cuts down on music, movie and software piracy by not allowing long term peer to peer servers to stay connected.