I've done tunneling/port forwarding/ANT config for years. I know how to get them work.<p>But fundamentally, I still don't understand why such issue exists.<p>Like, my (behind NAT, no public IP, whatever) device can visit any website or web services fine without any extra configuration. And obviously the servers of these services can reach me to send the content I need.<p>But then suddenly, if I want to reach my device from outside, I need all these extra stuff. What's the difference?<p>(I understand this is a very, very dumb question. Forgive my ignorance!)