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What you're talking about is network access translation; the router has its own IP to the external and internal worlds, whereas each computer has an IP address assigned to them by the router -- they have NO external IP. The router is the only thing 'visible' to the world. When you send an FTP command to an IP address, its going to the only visible IP -- that of your router. It's sent via the ports listed above, and so its then the job of the router to say -- "Port 21 and 20.. these automatically get forwarded to Computer A."
Dunno if that helps, but that is how it was explained to me long ago..
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"I'm typing on a computer of science, which is being sent by science wires to a little science server where you can access it. I'm not typing on a computer of philosophy or religion or whatever other thing you think can be used to understand the universe because they're a poor substitute in the role of understanding the universe which exists independent from ourselves." - Willravel
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