I've tried that, both in Linux and my command prompt, it doesn't work.
However, I did try that whatstheirip.com again, instead this time I used my gmail instead of Hotmail and it worked. But, it gave me there external IP. Is there a way I can find their internal IP by having their external one
Edit: By internal IP, I did mean mac address.. finding posts on google are confusing when some people don't know what they're talking about.