|If the lease time is longer than a 24 hour period, shutting the modem off for that long may not help. Also, if like my ISP, the one in question tends to reserve an IP to a specific MAC address, you could leave it shut off beyond the lease period (regardless of how long that is) and you'd still get the same IP back again.|
If you need to change your external IP, the simple solution is spoof a different MAC address. Recently I replaced the combo device given to me by my ISP when I first joined them (the wireless side of it was dieing) and replaced it with just a modem and bought my own router. I spoofed the MAC from my combo unit on my new router and got the same external IP as soon as I connected it.
Again, why would you want to change it? The only reason I can think of is because your current address has been blacklisted by a forum or as a source of Spam email or to circumvent some other restriction; changing the IP address won't solve the underlying problem.
If the above is the case, changing your IP address isn't likely to help much. Lets say this is an email spammer and they've been blacklisted, most folks blacklist the entire domain....so changing your IP won't help in that instance.
Even if they only blacklisted the one IP, if you change your IP and go back and spam the same domain again, then they certainly will blacklist the entire domain and they'll contact the offenders ISP to let them know. In fact, they'll probably contact the ISP the first time around an d let them know.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.