Restrict visitors to a site by IP address

Question

Not sure if this is the right place to ask this question but thought I would give it a shot.
I need to build a small website to collect entries to a sweepstakes. My client wants me to use an Internet application form service (Wufoo.com) to create the entry form that visitors will fill out for a chance to win a prize. Which is fine, I've used Wufoo many times but Wufoo will not do this thing I need to accomplish.
Just for the sake of offering all information, the site is hosted using a Virtual Private Server. (Debian 4 OS, Apache Web Server 2.0, PHP 5.0.4, MySQL 4.1.14)
Since my client wants to only allow visitors within the United States to enter the sweepstakes I thought it would be best to somehow only allow visitors with US IP addresses access to that page and also redirect visitors that attempt to access the form/entry page and send them to another page that says, sorry but the sweepstakes is for US visitors only.
There are probably a ton of ways to accomplish this but I can't figure it out.
In the interest of brevity, I won't list what I've learned, tried, and certain specific questions I've had because maybe someone out there has a solution that is completely different from what I've found. Thanks in advance.

Answer 1

There are two kinds of potential visitors to your site. 1) Those that know how the low-level IP works and 2) those that don't. You only care about people in group 2 because you are powerless to stop those in group 1. All you can do with those in group 1 is to refuse to send the winnings to a non-US address.
For group 2, you just need to be a bit sneaky. Record the name, address, and IP address of every visitor. Allow non-US addresses on the entry form. Then, after they click submit, record the address and IP address. Verify that the provided address is in the US. If not, display the "sorry" page.
There may be some people who are in group 2 but are on their way to group 1. At this point, they may try to enter a US street address but you will refuse to accept their entry because you have already flagged their IP address as being non-US.
I think is this your easiest, most fool-proof way of doing what you want. Let your users self-select. You can find out which IP addresses are assigned to non-US locations, but you would need to use the full ARINdatabase. It is possible, but difficult. Plus, there are lots of US-based proxies available that would defeat any ARIN or Geolocation scheme because they are, in fact, in the US.