Gerber is correct that a dynamic IP address is "leased" for the duration of a logon session, or for a preset interval of time. IPSs charge more for a fixed IP address, often MUCH more because that IP address has to be dedicated and hardware resources must be permanently allocated to that IP address. During the course of a day a given dynamic IP address may be used by multiple users thus reducing the ISP's physical hardware requirements. There is also an assumption on the part of ISPs that a fixed IP address is likely to be used for web hosting and the probability is it will use a lot more bandwidth.
Artie, I think you may be confusing NAT (Network Address Translation) with a dynamic IP address. NAT is a technology used in routers and does provide some security on local area networks. Rather than my attempting to describe NAT in detail I will refer you to
this Wikipedia article.
