I have a novice understanding of NAT and the ways a router can present multiple (internal) IP's to the world using a single (external) IP. But I'm struggling to understand a pattern I have found across three different networks. In each case, I browse to one of the web sites that tells you what your ip address is and different computers within a network all display the same ip address from the web site. From what I've learned about NAT, if they're all showing the exact same external IP, then the router is probably using port addressing to distinguish the different computers. But from what I understand about HTTP, it uses port 80, so the router would not be free to multiplex that value. Is there another NAT mechanism I'm missing that would allow multiple internal IP's to share an identical external IP?
Any help, or references to explanations, greatly appreciated.