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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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).
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