Hello,
The whatismyip.com will not show you your private IP address if you are behind NAT. On the contrary, the site will show you what is your public IP by simply looking at the source IP address of your request. After all, it would not make much sense telling you what is your private IP as you can always find it out directly on your PC. It is the public address after NAT that is often not known to ISP customers as it may change every now and then, and the whatismyip.com shows you what is your current public IP.
The FF:FE word added into the MAC address is used to inflate the 48-bit MAC into a 64-bit EUI address. The FF:FE word is specifically reserved to designate that a particular EUI-64 was created from a 48-bit MAC.
Best regards,
Peter