LinuxQuestions.org - how can a host outside a private network know the actual sender's ip ??

hi everyone,
we have a private class B network 172.31 subnetted. To connect to the internet we use some proxy servers connected to a router n then to the Internet.
I am a networking newbie and earlier i thought when a http request is sent using a proxy , the packet might be sent from the other i-face of the router i.e connected to Internet which has some ip other than 172.31 , but when i was browsing ip2location.com i found that it was displaying my private ip address n obviously that one of the router's..

how can they manage to know about a host on a private network??
unaware of much about PHP n web designing,firstly i thought they might be using some applet kind of thing in their HTML source code n know the private ip by running appropriate prog. on the client side.. but even that wasn't true.. here is a segment of the html source of that web page

Code:

 <TD class="fontgraysmall" width="65%">

 <span id="Livedemo1_lblIpAddress"

  class="fontgraysmall">172.31.85.140</span></TD>

which is simple html n nothing else.. could someone plzz clarify how do they manage to do this??

thanx in advance...
vijay