FTP Definition: How File Transfer Protocol Works

Updated 6 November 2026 · Standard formula, shown below

This page explains the File Transfer Protocol (FTP), the standard network protocol used to transfer files between a client and a server on a computer network. You’ll learn what FTP does, how its commands and data channels work, and how it compares to modern alternatives like SFTP and FTPS.

What Is FTP?

FTP stands for File Transfer Protocol. It is a standard network protocol defined in RFC 959 (1985) that allows a client to upload, download, delete, rename, and list files on a remote server. FTP uses a client-server model: the client initiates a connection to a server, authenticates (often with a username and password), and then issues commands to manipulate files. The protocol is stateful—it keeps track of the user’s current directory and authentication state during the session.

FTP was designed for a time when networks were less hostile. It sends data and credentials in plain text unless explicitly secured with TLS (FTPS). That is why modern practice often replaces plain FTP with SFTP (SSH File Transfer Protocol) or FTPS. However, FTP remains widely used for legacy systems, embedded devices, and bulk file transfers where security is handled at a higher layer.

How FTP Works: Control and Data Connections

FTP uses two separate TCP connections: a control connection and a data connection.

Control connection – Usually on port 21. The client sends commands like USER, PASS, LIST, RETR (download), and STOR (upload). The server replies with status codes (e.g., 230 for “logged in�, 550 for “file not found�).
Data connection – Used for the actual file contents or directory listings. This connection can be established in two modes: active or passive.

In active mode, the server connects back to the client’s IP address on a random port the client has specified. This often fails behind firewalls and NATs. In passive mode (the default in most modern clients), the client opens a second connection to a port the server provides, which works better with firewalls. Most FTP clients default to passive mode for this reason.

FTP Commands and Responses

FTP commands are short ASCII strings. Some common ones:

USER – send username
PASS – send password
PWD – print working directory
CWD – change directory
LIST – list files in the current directory
RETR – download a file
STOR – upload a file
DELE – delete a file
QUIT – end the session

The server replies with three-digit codes. For example, 220 means “service ready�, 331 means “user name okay, need password�, and 226 means “closing data connection, request successful�. These codes are standardized, so clients can interpret them without parsing human-readable text.

Active vs. Passive Mode: Which to Use?

The choice between active and passive mode is a common source of FTP connection failures.

Active mode – The client sends a PORT command telling the server which port to connect to. The server initiates the data connection back to the client. This fails if the client is behind a firewall that blocks inbound connections.
Passive mode – The client sends a PASV command. The server replies with an IP and port, and the client initiates the data connection. This works through most firewalls because the client only makes outbound connections.

For most users, passive mode is the right choice. It is the default in FileZilla, WinSCP, and most web browsers that support FTP. If you are setting up a server, you must allow a range of ports for passive data connections and configure your firewall accordingly.

FTP vs. SFTP vs. FTPS

People often confuse these three. They are not the same.

FTP – Plain text, unencrypted. Credentials and files can be intercepted.
FTPS – FTP over TLS/SSL. It adds encryption to the standard FTP protocol, but still uses two connections and can have compatibility issues.
SFTP – Not FTP at all. It is a file transfer subsystem of SSH (Secure Shell) that uses a single connection on port 22. It is more secure and simpler to firewall.

If you are starting a new project, SFTP is generally recommended because it is more secure and easier to manage. FTPS is useful when you must interoperate with legacy systems that only support FTP. Plain FTP should be avoided for any sensitive data.

Common FTP Problems and Solutions

Here are typical issues users face:

Connection timeout – Often caused by the server not listening on port 21 or a firewall blocking it. Check the server’s IP and port.
Active mode fails – Switch to passive mode in your client settings.
“530 Login incorrect� – Wrong username or password. If you are using anonymous FTP, try anonymous as the username and your email as the password.
File transfer stalls – May be due to a passive port range not being open on the server. Configure the server to use a specific port range and allow it in the firewall.
Binary vs. ASCII mode – Transferring a binary file (like an image) in ASCII mode can corrupt it. Most clients auto-detect, but you can manually set binary mode.

FTP Security: Why It Matters

Plain FTP sends your username and password in clear text. Anyone on the same network can sniff them. This is why FTPS and SFTP exist. If you must use plain FTP, limit it to non-sensitive files or run it only on a trusted internal network. For any administrative access, always use an encrypted protocol.

Also, FTP servers should be kept up to date with security patches. Many legacy FTP servers have known vulnerabilities. If you are setting up a new service, prefer SFTP (via OpenSSH) or FTPS with a valid certificate.

How to Choose an FTP Client

There are many FTP clients, both graphical and command-line. The choice depends on your platform and needs.

FileZilla – Free, cross-platform, supports FTP, FTPS, and SFTP. Good for beginners.
WinSCP – Windows only, integrates with Windows Explorer, supports FTP, FTPS, and SFTP.
Cyberduck – Mac and Windows, supports many protocols including FTP, SFTP, and cloud storage.
Command-line ftp – Built into most operating systems, but lacks encryption and is hard to script.

When choosing, make sure the client supports the protocol you need (FTP, FTPS, or SFTP) and has the features you want, like synchronization or site manager.

Frequently Asked Questions

Is FTP still used in 2024?

Yes, though its use is declining. Many legacy systems, web hosting services, and embedded devices still support FTP. However, for new deployments, SFTP or FTPS is strongly recommended due to security concerns.

What is the difference between FTP and HTTP for file transfer?

HTTP is designed for web pages and can also transfer files, but FTP is more efficient for large batches and offers better file management (rename, delete, list). FTP maintains a persistent control connection, while HTTP is stateless. For one-off downloads, HTTP is simpler; for managing a remote directory, FTP is more suitable.

Can FTP be used over the internet?

Yes, but you must configure your firewall to allow the control port (21) and a range of passive data ports. Also, because FTP is unencrypted, it is not recommended for sensitive data over the internet.

Why does my FTP connection work but then times out?

This often happens in active mode when the server cannot connect back to your client. Switch to passive mode. Also, check that the server’s passive port range is open in its firewall.

What does “anonymous FTP� mean?

It means you log in with the username anonymous and your email as the password. Many public FTP servers offer this to allow anyone to download files without a personal account.

How do I transfer files securely with FTP?

Use FTPS (FTP over TLS) or SFTP (SSH File Transfer Protocol). Both encrypt your credentials and data. Plain FTP should be avoided for any sensitive information.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.