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An FTP server can (and regularly does) handle more than one client at a time.
FTP exists on web servers on the internet and is the most common method of publishing a web site.
FTP is also used as the most common method of copying files around office local and wide area networks.
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For most modern FTP users, the inner workings of FTP are completely hidden. We all just use easy to use FTP clients and connect up to hidden FTP servers.
Behind the scenes, the FTP client communicates with an FTP server using a predefined language over a control ASCII channel. Data channels are automatically created by both the FTP server and client as and when they are needed to transfer files or directory listings.
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The two main modes that FTP clients and servers communicate are passive and non-passive.
In non-passive mode (or active mode), when a client initiates a transfer either TO or FROM the server, the client itself tells the server which port to contact it on and the FTP server then connects to the waiting client for the data transfer.
Non-passive mode (active mode) is the default for most systems.
In passive mode, when a client initiates a transfer either TO or FROM the server, the server informs the client which port to contact it on for the data trasfer and waits for the client to connect up before sending or accepting the data.
Passive mode is regularly used by people using an FTP client from behind a firewall.
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