1. This is very similar to the way you get a new
external IP whenever you dial-up to the internet (some folks still do that) or disconnect your ADSL for a few minutes
2 (less scrupulous net citizens use this "feature" regularly!).
2. Although it's usual to get a new external IP when you reconnect your DSL, it's not guaranteed. With some ISP's it happens rarely, with others you need to disconnect for a few minutes or more, with some ISP's, you always get a fresh IP with each connect, nomatter how quickly you do it. One thing is certain, though; unless you are
paying them for a static IP, your current external IP address
will change!
3. Remember, your external IP, and your local computer's IP, are two totally separate things (well, in this context). Almost no one needs a static IP for their
external connexion, but almost everyone who wants to run peer-to-peer applications, ftp servers, and so on,
will need a static IP for their
local computer. The former is supplied by your ISP, the latter is down to you.