That's almost always the case when you are on the internet behind a 'gateway' device like a cable or DSL transceiver(some people call them modems). Your internal network is always hidden from the internet, you can have ports opened up, sending certain kinds of transmissions to internal machines, or a terribly configured endpoint device that allows anything to go anywhere. Still, even in those cases your internal network still isn't visible to others on the internet. Your public(WAN)IP is all anyone will see.
I just wondered how secure that masking is.