I'm logged into my Google account, and it shows a certain IP address as where I'm logged in from. Checking ipconfig shows the same IP address as being the computer I'm on. The proxy will of course show it as being the actual source, but Google is smart enough to show the IP address that the proxy says it's proxying for. AFAICT, there's no NAT, but there is a firewall blocking traffic that doesn't run through the proxy.