"What will happen if the ISP of the client allocates dynamic IP for the client whenever client connects to the internet?"
You are right vijay6 it is a good concern but one would have to turn off and on their modem and hope to get a different IP just to vote again. My ISP allocates dynamic IP addresses as well, and I do not know if it is the same for most ISP but in over 10 years I think my IP address changed three or four times...but then again my modem is always on.
So you brought a valid point, and there are other methods to consider:
One could to try to capture the MAC Address of the user, but that can be falsified as well.
Cookies is another way, but cookies can be deleted.
Getting voters to register and login before voting, but again one can register multiple times.
There is a lot to consider but for the sake of simplicity I think this is good enough for the purpose of a poll (unless of course you are electing a new president :)
There is probably a better way to do this that someone knows out there but that was my two cents
P:oD
p.s.
cool name "vijay6", you should get a proper avatar