Most likely Amazon are running a load balancer, so that if you have multiple machines running the same http server, the load gets shared between them automatically. It also allows Amazon to save on public routeable IP addresses, which as we know are becoming a scarce resource.

As others have said, you need to examine the headers, to look for one that contains the original IP address. Most likey it will be HTTP_X_FORWARDED_FOR, but it is entirely possible that Amazon have decided to do their own thing.

The other thing to be aware of is that your clients will most likely be behind a proxy from their ISP, so you are unlikely to see their real IP address. If you are lucky, the proxy at the client end will also have put the IP address in a special header. The other possibility is that both proxies put the IP addresses in HTTP_X_FORWARDED_FOR, to create a comma separated list of IP addresses.