Because the request sent by the internal network host to the external network will pass through the router, and the router will forward it.

Such as: router public network IP188.188.188.438+088.

Your intranet IP 192 438+068 438+0.8.

If you initiate a request192.168.1.8: 8970-> opposite IP: port.

After receiving it, the router will record your intranet IP and port (establish routing table) and enable the new port to request the other party.

Actually, it may be:188.188.188.188: 6780-> the other party's IP: port.

By the way, this is called network address translation protocol, which is NAT forwarding.