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About FTP

File Transfer Protocol (FTP) is an Internet protocol that runs in the application layer which is a standard for sending computer files between machines on an Inter-network. FTP is one of the earliest Internet protocols developed, and is still used today to download and upload computer files between an FTP client and an FTP server. An FTP Client is an application that can issue FTP commands to an FTP server, while an FTP server is a Windows Service or daemon that runs on a computer that responds to commands from an FTP client.

FTP commands can be used to change directories, change the sending mode between binary and ASCII, upload computer files to the FTP server, and download files from the FTP server. An FTP server is accessed using Universal Resource Identifier (URI) using the ftp://servername format. The FTP client can contact the FTP server by opening the URI. FTP uses the Transmission Control Protocol (TCP) protocol for data communication between client and server, so between the two components a communication session will be created before data transmission begins.

Before making a connection, TCP port number 21 on the server side will "listen" to an attempted connection from an FTP client and will then be used as a control port to
  1. Make a connection between the client and server,
  2. Allow the client to send an FTP command to the server and also
  3. Returns the server's response to the command.

Once the control connection has been made, the server will start to open TCP port number 20 to form a new connection with the client to send the actual data that is being exchanged while downloading and transferring. FTP only uses the standard authentication method, which uses a username and password sent in an unencrypted form. Registered users can use their username and password to access, download, and upload the files they want. Generally, registered users have full access to several directories, so they can create files, create directories, and even delete files. Users who have not been registered can also use the anonymous login method, namely by using anonymous user names and passwords that are filled in using e-mail addresses.

Source: https://en.wikipedia.org/wiki/File_Transfer_Protocol