Server Message Block is the language your computer uses to share files and printers over a network

Server Message Block (SMB) is a communication protocol — a set of rules that lets computers talk to each other over a network. When you open a shared folder on another computer, print to a network printer, or access files stored on a company server, SMB is doing the work behind the scenes. It handles the back-and-forth conversation between your machine and the other computer, making sure the right file gets sent to the right place.

You do not need to do anything to use SMB. It runs automatically on Windows, Mac, and Linux computers. The protocol has been around since the 1980s and is still the standard way that computers in offices and homes share resources with each other. Understanding what it does helps you troubleshoot network problems and recognize security warnings when they appear.

Key Takeaways

  • SMB is the protocol that handles file sharing, printer sharing, and network communication on Windows, Mac, and Linux machines.
  • When you see a shared folder icon or connect to a network printer, SMB is managing that connection in the background.
  • SMB runs on port 445 on modern networks and older versions ran on port 139, which is why you may see these numbers in network settings.
  • Older versions of SMB have known security weaknesses, which is why Windows and other systems regularly release updates to patch them.
  • Disabling SMB entirely will break file and printer sharing on your network, so the safer approach is to keep it updated and use strong passwords.

How SMB moves files between computers

When you click on a shared folder in Windows File Explorer or Finder on Mac, your computer sends an SMB request to the other computer asking for access. The other computer checks whether you have permission, then either grants or denies the request. If you have permission, SMB handles the actual transfer of the file data, breaking it into packets and reassembling it on your end.

The same process happens when you print to a network printer. Your computer does not send the document directly to the printer. Instead, it sends an SMB message to a print server or to the printer itself, saying "here is a document, please print it." The printer or server receives the message, queues the job, and sends back a confirmation. This is why a network printer can handle multiple print jobs from multiple computers without getting confused.

SMB also handles authentication — the process of proving who you are. When you connect to a shared folder, you may be asked for a username and password. SMB encrypts that information and sends it to the other computer, which checks it against its list of authorized users. If the password matches, the connection is allowed. If it does not, the connection is refused.

The difference between SMB versions and why updates matter

SMB has gone through several versions over the decades. SMB1, released in the 1980s, is the oldest and slowest. SMB2, released in 2006, was much faster and more secure. SMB3, released in 2012, added encryption and better performance. Most modern computers use SMB2 or SMB3 by default.

The reason version numbers matter is security. SMB1 has known vulnerabilities — weaknesses that attackers can exploit to break into your computer or steal files. In 2017, a major ransomware attack called WannaCry spread across the world by exploiting an SMB1 vulnerability. After that attack, Microsoft and other companies began disabling SMB1 on new computers and pushing users to update.

When you see a Windows security update that mentions SMB, it usually means Microsoft has found and fixed a vulnerability in SMB2 or SMB3. These updates are important because they close the doors that attackers might use. Delaying updates leaves your computer exposed, especially if you are on a shared network where other computers might be infected.

When you see SMB errors or warnings

If you try to connect to a shared folder and see a message like "SMB1 is not supported" or "the connection is not secure," it usually means one of two things: the other computer is using an old version of SMB, or your computer has SMB1 disabled for security reasons. The fix depends on which computer is the problem.

If the other computer is a printer, router, or older device that only speaks SMB1, you have two options. You can re-enable SMB1 on your computer (which lowers your security), or you can update the firmware on the other device (which is safer but may not be possible). If the device is very old and cannot be updated, re-enabling SMB1 only for that specific device is sometimes the only choice.

If you see a warning that SMB is "not encrypted" or "not signed," it means the data traveling between your computer and the other computer is not protected. This is a real risk if you are on a public or untrusted network. On a home or office network with a password-protected router, the risk is lower, but you should still update to SMB3 if both computers support it.

SMB on home networks versus office networks

On a home network, SMB usually works without any setup. When you turn on a computer or printer, it broadcasts its presence to the network. Other computers see it and add it to the "Network" section of File Explorer or Finder. You can click on it, enter a password if one is set, and start accessing files.

On an office network, SMB is usually managed by a network administrator. The administrator sets up a central server, creates user accounts, and controls which folders each person can access. When you log in to your work computer, it automatically connects to the shared folders you need. You do not have to type in passwords for each folder because your login credentials are already verified.

The security model is also different. A home network usually relies on a single password for each shared folder. An office network uses more complex permissions — you might have read-only access to one folder, edit access to another, and no access to a third. SMB handles all of this behind the scenes, checking your permissions every time you try to open a file.

Why SMB is still used even though it is old

SMB has been around for over 40 years, which makes it ancient in technology terms. You might wonder why computers still use it instead of something newer. The answer is that SMB works, it is reliable, and changing it would break millions of existing networks and devices.

Every printer, network storage device, and shared folder in the world relies on SMB. Replacing it would mean updating all of those devices at once, which is impossible. Instead, the technology has evolved. SMB3 is much more secure and faster than SMB1, but it still speaks the same basic language. A modern computer running SMB3 can still talk to an older device running SMB2, even if the connection is slower or less secure.

This backward compatibility is both a strength and a weakness. It means your network keeps working even when devices are different ages. It also means that old, vulnerable devices can stay on the network and potentially cause problems. This is why security updates and disabling old versions of SMB are so important.

Frequently Asked Questions

Is SMB the same as Samba?

No, but they are related. Samba is an open-source implementation of SMB that runs on Linux and Unix systems. It lets Linux computers share files with Windows computers using the SMB protocol. Samba translates SMB messages so that Linux can understand them, making it possible for different operating systems to work together on the same network.

Should I disable SMB on my computer?

Disabling SMB entirely will break file and printer sharing on your network, so it is not recommended unless you do not use any shared resources. A safer approach is to disable only SMB1 (which is usually already disabled on modern Windows) and keep SMB2 and SMB3 enabled. Make sure your system updates are current, and use strong passwords on shared folders.

What is the difference between SMB and NFS?

NFS (Network File System) is another protocol for sharing files, but it is more common on Unix and Linux systems. SMB is the standard on Windows networks. The two can coexist on the same network, but they do not talk to each other directly. If you need to share files between Windows and Linux, Samba (which uses SMB) is usually the bridge.

Why do I need a password for a shared folder if my network already has a password?

The network password protects access to the internet connection, but it does not control who can see files on individual computers. A shared folder password is a separate layer of security that lets you decide which people on your network can access which files. You can have a shared network but still keep some folders private by setting a password on them.

Can SMB be used over the internet, or only on local networks?

SMB is designed for local networks and is not secure over the internet without additional protection. If you need to access files remotely, you should use a VPN (Virtual Private Network) to encrypt the connection, or use a cloud service like OneDrive or Google Drive instead. Exposing SMB directly to the internet is a common way for attackers to break into computers.