A and B octet IP randomizer needed

I am in need of a program that will be able to somehow fake the A and B octet of the current IP address... (A.B.C.D) so, the first 2 sets of numbers... I am using dialup to change IPs often and automatically, but A and B octets are beginning to be blocked, so if I could randomize those and continue to use the constantly changing C and D octets provided from the dialup ISP, I think everything should run smoothly.. Program should be able to run automatically and have the option to set the time intervals in which the A and B octets are randomized... If this sort of thing is possible, please PM me with some info on how you could help, and a quote... Thanks