finding the ip adres of the visitor of a web site.

Hi to all IPv6 users,

Does anyone know how can i learn the ip adres of a client that is connected
to my web site, and whether he/she uses IPv6 address or not, like
www.kame.net and www.sixxs.net does at the bottom of their sites. Is there
any open source script for this. I will also appreciate if someone can tell
me how it is done, rather that the code itself...

Best,

Mustafa Sahin