Suppose a router device connect a private network and the Internet, and doesn't have NAT or NAPT features.

If some application on a host inside the private network sends some packets to a host in the Internet, can the host in the Internet send some packets as a response back to the host in the private network? If yes, how?

For example, my computer is in a private wifi network behind a router. Is it correct that most routers by default don't have NAT or NAPT set up? (I think yes, because I heard that to make a server inside a private network to be publicly accessible, I need to enable NAT/NAPT on the router for the network.) So assume my wifi router doesn't have NAT or NAPT set up. How can two computers sharing the same wifi network can receive webpages from different public web servers on the Internet? In other words, how can the router know which computer in the wifi network to send HTTP responses from the different web servers to, without NAT or NAPT?


marked as duplicate by Ron Trunk, user36472, Ron Maupin Mar 29 at 14:16

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Private addresses (by agreement) are not routed over the public Internet. So the reply packet would never reach the sender. In addition, most ISPs filter private addresses as source addresses (at least, they should). so the packet would never reach the host in the first place.

Let me reiterate (because it still seems to be a point of confusion) that all of the above is based on policy agreements. There is noting inherent about private IPs that prevents them from being routed over the Internet. We just all agree not to do it.

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