WAN stands for Wide Area Network. The WAN-Port is the uplink to the internet. While the LAN-ports (Local Area Network) will connect to your computer and other devices, the WAN-Port needs to be connected to the wall or the modem that your ISP has provided.

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Firewall Port Exceptions | Opening Firewall Ports

Which ports are used by TeamViewer? Oct 25, 2019 Port Triggering Help - TP-Link

EDIT, for clarification: Let's say that my server listens TCP port 80. A client makes a connection and port 56789 is allocated for it. Then, I discover that this connection is undesired (e.g. this user is doing bad things, we asked them to stop but the connection didn't get dropped somewhere along the way).

Understanding Port Forwarding- Beginners Guide Apr 11, 2020