Edit: I had an example of displaying the IP address back to the user from my server, but it went over capacity in a few short minutes, so I took it down and removed the link. I am sure everyone gets the point, specially with a lot of the other examples provided by other HN users below.
https://hulbee.com/?query=perl%20-e%20'print%20%22%3CIMG%20S...
Edit: I had an example of displaying the IP address back to the user from my server, but it went over capacity in a few short minutes, so I took it down and removed the link. I am sure everyone gets the point, specially with a lot of the other examples provided by other HN users below.
Here is a screen shot for future reference: http://imgur.com/PkAGhqn