How are IP check sites ID my internal IP address? I want it hidden.

Question

Hi all, 
 
 I'm not a noob with networks or firewalls, in fact I started my career in these areas long, long ago. Anyway, I was checking some outbound traffic today to ensure it was showing the gateway IP using whatismyip[dot]com, and I was surprised to see that it not only showed my gateway external IP, but also my internal host IP. I don' t understand because I have "Rewrite source address (masquerading)" enabled for the default policy which should be the only coming into play here. 
 
 I suspect it is just that site reading the IP via Javascript or something because none of the other sites are showing it? 
 
 Can someone help me address this? I see it as a major security issue if it is in fact something the whole world can see. 
 
 Thanks!

Mark Whitworth · Answer

Checking on this, I found that if I use WebRTC Network Limiter I can deny it access to my private IP. Looks like this is a behavior from Chrome and Firefox now allowing WebRTC in the browser for STUN calls and such. Not sure if there are better options even for this, but wanted to report in.

Aditya Patel · Answer

Hello, 
 It does seem some specific vendors are using the same implementation. 
 https://security.stackexchange.com/questions/94783/why-is-my-internal-ip-address-private-visible-from-the-internet