Get the client IP is easy:
HttpContext.Current.Request.UserHostAddress
will give it to you directly.
Getting the website IP address is only slightly more complex:
http://www.howtogeek.com/howto/programming/get-ip-address-from-dns-hostname-in-c/[
^] - it's in C#, but it's pretty obvious!
You cannot change the client IP address - it is not issued to you or by anything you have control over. If nothing else, if you could change it, you wouldn't be able to respond to the user...