Revealing my Internal IP

Hi,

I am inside a company network. When I am browsing Internet, my internal IP is getting NATted and packets are sent outside. It is easy for web programmers to get my public IP through which it is NATted. But there are some websites which displays my public IP and even my internal IP. I went through the source code and cant find any code which tries to retrieve my internal IP. How exactly it is getting revealed???