This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
HttpServletRequest.getRemoteAddr returns the IP address from where the request originates. You can compare that with the list of approved addresses, and return an error (maybe 403 - SC_FORBIDDEN) if it's not in that list.