Like WindSpeed said, Or they own the IP's ready, Its like DHCP on the ISP side but instead of releasing those IP's every few hours. They get reserved to you/your host till you end the contact or remove those IP's from your account. The game hosts and so on do not have modems, it works on the same principle as LAN's. The ISP's already own a range of IP's so they are able to give those out on demand.