Why would you want to do that? I hope it's not for anything malicious but for informational purposes you may try an IP stealer one of the admins. here happen to host one. [URL="http://www.syntax******.info"]http://www.syntax******.info[/URL]
make an account with his site and go to software/tools and select IP stealer and create a redirect link. anyone that you send that link to that clicks it will fore-fit their IP address and once you have it you can type in whats my IP in the google search and click a site the rest is self explanatory. I do however s***est that you go to tiny url and create a custom url instead of using the syntax****** url that is kinda a dead give away.