What Is an FTP Server? The Meaning, How It Works, and What It Is Used For

The request usually arrives as one line in a ticket: "The customer wants to send us files. Can we get an FTP server?" Someone forwards it to IT, IT forwards it to the newest person in IT, and the newest person types the phrase into a search box. If that is how you got here, welcome. The phrase covers a small idea and a surprising number of decisions.

This article explains what an FTP server is in plain English. It covers what the server does with each connection, how it works on the network, what organizations use one for, how it differs from a file share and from its encrypted relatives, FTPS and SFTP, and which free and paid server software exists. It ends with how to connect to one, what you need to run one, and a setup sheet you can copy straight into that ticket.

FTP Server Meaning: The Program, the Protocol, and the Machine

That is the whole FTP server definition, and it packs three things into one phrase. People use the phrase for any of them. The program is the software that listens and answers: the FTP feature built into Windows, a free server, or a commercial product. The protocol is FTP itself, the rules both sides follow, whose current specification dates from 1985 and is still followed today. The machine is the computer the program runs on, which is what "the FTP server is down" usually means. The rest of this page is what follows from the definition: who connects, what the server checks, and what can go wrong between the two.

What an FTP Server Does With Each Connection

Every FTP server does the same four things for every connection, whatever software it runs. It checks who is connecting, opens that user's home folder, applies the rights the administrator set, and writes down what happened. The diagram shows the people and machines that typically connect, and the work the server does on their behalf.

It checks who is connecting. The client sends a user name and a password, and the server compares them with its list of accounts. Some servers keep their own list. Others ask Windows or a directory service such as Active Directory. A wrong password gets a refusal and, on a well-run server, a line in the log.

It opens that user's home folder. Each account is mapped to a starting folder on the server's disk. A partner called Acme might land in /partners/acme and see nothing above it. This mapping is what separates an FTP server from a shared drive: each user gets a small world of their own.

It applies read and write rights. The administrator decides whether each user can download, upload, delete, rename, or create folders. A supplier who only sends invoices needs upload and nothing else. A public download area needs read and nothing else. The most common mistake is giving everyone every right because that was the default, which works fine right up until someone deletes the folder everyone else depends on.

It logs what happened. Who logged in, from which address, and which files moved. Nobody reads the log on a good day. On a bad day it is the only witness.

The other side of the conversation is the FTP client, the program that connects. It can be a person's file transfer program, a script, an office scanner, or another company's system. The server waits and the client calls. Which of the two you need is the first decision in choosing FTP software for Windows, and it is not always the one the ticket asked for.

How an FTP Server Works

FTP is older than the web, and its design shows its age in one important way: it uses two separate connections. The first, the control connection, goes to the server's port 21 and stays open for the whole session. Commands travel over it, such as "log me in", "list this folder", and "send me that file", and the server's numbered replies come back, such as "230 Login successful". The second, the data connection, is opened fresh for every folder listing and every file, carries that one thing, and closes.

Who opens the data connection depends on the mode. In active mode the server connects back to the client, traditionally from port 20. In passive mode the client makes the second connection as well, to a port the server announces from a range the administrator has set aside. Passive mode is the norm because it gets through firewalls and home routers, which tend to object to strangers calling in. The two modes, and why one fails where the other works, are explained in active vs passive FTP. The two connections themselves are covered in FTP control and data connections.

The practical consequence is that an FTP server needs more than one firewall rule: port 21 for the conversation and a range of ports for the data. Open only port 21 and users can log in, read the welcome message, and then wait forever for a folder listing that never arrives. It is the most reported FTP problem there is, and the cause is nearly always the same firewall rule.

The full list of ports for FTP, FTPS, and SFTP, and the firewall rules each one needs, is in our port reference. The quick answer to the most common question is in what port is FTP?

FTP Server Examples: What Organizations Use Them For

FTP has been declared dead regularly for a long time. It keeps turning up anyway, because a protocol that every operating system, script, and office device understands is hard to replace. These are the jobs FTP servers do most often.

• Exchanging files with partners. A supplier uploads price lists every night, a bank sends statements, a payroll provider collects time sheets. Each partner gets an account and a folder. This is the most common reason a business runs an FTP server.
• Receiving files from devices. Office scanners, copiers, security cameras, and lab instruments often have exactly one way to send a file anywhere: FTP. "Scan to FTP" is how many of them deliver documents into a shared folder.
• Updating websites. Web hosts have long given customers FTP access to the folders behind their site. Developers upload changed pages and images with an FTP client.
• Moving files between systems on a schedule. One system exports orders, a script uploads them, and another system picks them up. Nobody is at the keyboard. The client is a job that runs at two in the morning.
• Backups and archives. A server copies its nightly backup to a second location over FTP, because the backup tool supports it and the second location has plenty of disk.
• Public downloads. Software vendors, universities, and open-source projects ran anonymous FTP sites for years. Most have moved to web downloads, but some of the sites, and the habit, remain.

Northgate Retail, a chain of a hundred stores, is the whole list in one company. Every store's scanner sent receipts to a server at head office over FTP. Forty suppliers dropped invoices into folders of their own, and the website team still pushed updates the same way. Nobody had planned an FTP strategy. Each use had arrived on its own, one ticket at a time, and the server had quietly become one of the most important machines in the building. When it was finally moved to new hardware, the migration plan ran to nine pages. Eight of them were the list of who connected to it.

The pattern is worth noticing: whatever an FTP server was set up for, it ends up doing more. That is the main reason to choose one that can grow with the requests, which comes up again below.

FTP Server vs File Server, FTP Service, and FTP Storage

Several phrases get used as if they meant the same thing. They overlap, but not completely.

FTP server vs file server. A file server is any machine whose job is to store files for others. On a Windows network that usually means shared folders reached through Windows file sharing, the drive letters people map in File Explorer. Sharing works well inside one office network and badly across the internet, where it is usually blocked, and rightly so. An FTP server is a file server that people reach with FTP instead. FTP is built to cross networks, works with any operating system, and keeps each user inside their own folder. Many organizations run both: shares for the staff in the building, and an FTP server, or its encrypted equivalent, for everyone outside it.

FTP service. The phrase means two different things depending on who says it. To a Windows administrator, "the FTP service" is the FTP server feature built into Windows, which can be switched on and configured without buying anything. To a business buyer, an "FTP service" or "FTP as a service" usually means a hosted server that someone else runs and rents out by the month. The trade-offs between running your own and renting one are set out in hosted vs self-hosted file transfer servers.

FTP storage. People search for "FTP storage" or "FTP file storage" when they mean disk space reachable by FTP, usually from a hosting or storage provider. The storage is ordinary disk, and FTP is only the doorway. Check which doorways a storage plan actually offers before you sign up. Many have moved from plain FTP to encrypted alternatives, and some still offer only the plain kind.

For the rest of the vocabulary, from FTP site and FTP link to FTP user and FTP folder, see FTP terms defined in plain English.

FTP, FTPS, and SFTP Servers: The Security Question

Here is the part the original FTP design never considered: plain FTP encrypts nothing. The user name, the password, the folder listings, and the files all cross the network as readable text. Anyone in a position to watch the traffic can read the password as easily as the server does. That includes someone on a shared network, someone at a compromised router, and someone at the wrong kind of coffee shop.

Two protocols fix this. Their names cause endless confusion, because they differ only in where one letter sits.

• FTPS is FTP with TLS encryption added, the same encryption that protects HTTPS web pages. It keeps FTP's two connections and port 21 (or 990 in the older "implicit" style), and the server needs a certificate.
• SFTP is the SSH File Transfer Protocol. Despite the name, it is not FTP at all. It runs inside SSH on port 22, uses a single connection, and identifies the server with a host key instead of a certificate. An SFTP server is a kind of SSH server, and what else an SSH server on Windows can offer is a separate decision.

Neither works with the other's clients, and neither works with a plain FTP client set the wrong way. The full comparison is in SFTP vs FTPS, and what SFTP is covers the newer of the two from the beginning.

In practice, plain FTP is acceptable for public files on a trusted network, and for devices that cannot do anything else, kept on a network segment of their own. Anything with a real password or a real customer's data belongs on FTPS or SFTP. If a security questionnaire asks whether your server is "secure FTP", the questions it is really asking are listed in what makes an FTP server secure. Most partners now ask for SFTP by name, which is worth knowing before you choose software.

How to Connect to an FTP Server

To connect, a client needs four things from the server's administrator: the FTP server address, the port, a user name, and a password. The address is usually written as a URL such as ftp://ftp.example.com, or as a bare host name or IP address. If the port is not 21, it goes after a colon, as in ftp.example.com:2121. Keep the details together. This card is worth copying for every connection you are given.

FTP CONNECTION CARD
Server address:   ftp.example.com   (203.0.113.10)
Protocol:         FTP   /  FTPS (explicit)  /  SFTP
Port:             21    /  21               /  22
User name:        acme-upload
Password:         (sent separately, never in the same email)
Mode:             passive
Folders:          /inbound (we upload)   /outbound (we download)
Their contact:    name and phone number

On Windows there are three ways in without installing anything. File Explorer can open an ftp:// address as a folder. The ftp command works in a command prompt. For SFTP, current Windows includes the sftp command. All three are described in how to use FTP on Windows. Web browsers used to open FTP addresses too, but the major browsers have all removed that ability, as explained in FTP in a web browser.

For regular use, a dedicated client program is easier and, unlike the built-in tools, handles FTPS and SFTP as well. FTP software for Windows sorts the free and paid options by the job they suit. To check that a server works, and to decode the error messages when it does not, follow how to connect to an FTP server and test it.

FTP Server Software: Free and Paid

Half the people who search for "FTP server" want the software rather than the definition, so here is the software side in one table. There are four kinds. Which one fits depends on who connects and which protocols they insist on, and most partners now insist on SFTP.

"Free" covers the license, not the afternoon spent on it or the SFTP that a partner asks for six months later. The free kinds suit a single purpose with a few users. The paid kind earns its price when the server starts collecting uses the way Northgate's did. FTP software for Windows, free and paid sorts the clients and servers by the job they suit.

What You Need to Run an FTP Server

Running an FTP server takes three things: a computer that stays on, server software, and a network path that lets clients reach it. The computer can be a Windows server in your building, a Linux machine, a virtual machine at a cloud provider, or a hosted service that rents you all three at once.

The software comes in the four kinds compared above. Which kind fits depends mostly on who your users are and which protocols they insist on, and the decision is walked through in the Windows FTP server software guide.

The setup itself takes an afternoon. Install the software, create the accounts and folders, open the ports in every firewall between the clients and the server, and test from outside your own network. The detailed steps for Windows are in how to set up an FTP server on Windows. Before you start, fill in a sheet like the one below and attach it to the ticket. It turns a vague request into a checklist, and it is the document you will be glad of a year from now, when somebody asks why the server exists.

FTP SERVER SETUP SHEET
Purpose:            receive invoices from suppliers; scans from branch copiers
Host name:          ftp.example.com  ->  public address 203.0.113.10
Protocols:          SFTP (port 22) for partners
                    FTPS (port 21) for devices that cannot do SFTP
                    plain FTP only on the scanner network, never from the internet
Passive port range: 50000-50100  (open in every firewall on the path)
Accounts:           one per partner or device, never shared
Home folders:       D:\transfer\<account>   each user confined to their own
Rights:             suppliers upload only; scanners upload only; staff read + delete
Logging:            logins and file operations, kept 90 days
Owner / backup:     name, name
Review date:        ________

If files need to move on a schedule rather than when a person clicks, the client side matters as much as the server. Automating FTP downloads, sync, and triggers covers scripts, schedulers, and when automation software such as Sysax FTP Automation beats a batch file.

The Version to Tell a Colleague

An FTP server is a program that waits for connections on port 21, checks a user name and password, and lets each user upload and download files in their own folder, with the rights the administrator gave them. Organizations use FTP servers to exchange files with partners, receive scans from devices, update websites, and move files between systems on a schedule. Plain FTP sends passwords and files as readable text, so anything that matters should use FTPS or SFTP, and most partners now ask for SFTP. To use a server you need its address, port, user name, and password. To run one you need server software, a few firewall rules, and a setup sheet that says who is allowed to do what.

Frequently Asked Questions