FTP (File Transfer Protocol)
Protocol application layer of the TCP/IP stack is designed to share files on the Internet. Uses both independent connections over TCP for control and data transfer. Is one of the oldest Internet services but still widely used due to its simplicity and reliability.
The FTP server is listening on port 21 and request opens a single connection-FTP control session. When executing within that session, to transmit data to open additional connections. These connections can be either active or passive.
When you create the active connection (Active FTP) klent passes FTP server port number, IP address and port for the connection opens. The server connects to the specified port on the client's port 20 (FTP Data) and transfers the data across an established connection. Passive FTP is the opposite way. A client sends a request to the server to retrieve data, and the server returns to the client IP address and port number to connect. The client connects his port 20 (FTP data) to the specified port on the server and retrieves the requested data.
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