The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command.

第一个端口连接服务器的21端口，与主动FTP模式中提交PORT命令并允许服务器来回连它的数据端口不同，客户端会提交PASV命令。

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For instance, Web browsers generally bring HTML code into the system via port 80, while FTP programs transfer files through port 21.

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