FTP Server and Port Number

FTP (File Transfer Protocol) is a standard network protocol used for transferring files between a client and a server on a computer network. It operates on the client-server model, where the client initiates a connection to the server to transfer files.

Port Number

In computer networking, a port number is a 16-bit integer that is used to identify specific processes or services on a device, such as a computer or a server. Port numbers are used in conjunction with IP addresses to establish a connection between two devices. Each port number is associated with a specific protocol or service.

FTP Port Number

The FTP server listens for connections on port number 21. This means that the server is configured to receive incoming FTP connections on port 21. The server waits for clients to initiate a connection on this port so that it can establish a communication channel and transfer files.

Explanation

When a client wants to connect to an FTP server, it initiates a connection request to the server's IP address using port number 21. The server listens for incoming connections on this port and responds to the client's request. Once the connection is established, the client can send commands to the server to perform various FTP operations, such as uploading, downloading, or deleting files.

It is important to note that FTP uses two separate ports for communication. Port 21 is used for the control channel, where commands and responses are exchanged between the client and the server. The control channel is responsible for initiating and managing the FTP session.

In addition to the control channel, FTP also uses a data channel for transferring the actual file data. The data channel can use different port numbers, depending on the FTP mode being used. In active mode, the server initiates a connection to the client on a dynamically assigned port number. In passive mode, the client initiates a connection to the server on a dynamically assigned port number.

However, the default port number for the control channel, which is used for establishing the initial connection, is 21. This is why the correct answer to the given question is option 'B' - 21.