Server Message Block (SMB)

Definition of Server Message Block (SMB) in The Network Encyclopedia.

What is SMB (Server Message Block)?

A high-level file-sharing protocol jointly developed by Microsoft, IBM, and Intel for passing data between computers on a network. Microsoft Windows and OS/2 use Server Message Block (SMB). Many UNIX operating systems also support it.

How It Works

SMB is used between clients and servers to do the following:

  • Open and close connections between client redirectors and shared network resources
  • Locate, read, and write to files shared on a server
  • Locate and print to print queues that are shared on a server

SMB uses four types of messages:

  • Session control messages:
    Open or close a redirector connection to a shared resource on the server. The SMB redirector packages the requests sent to remote servers in an SMB-enabled network.


  • File messages:
    Used by the redirector to gain access to files on the server.


  • Printer messages:
    Used by the redirector to send data to a print queue and get status information about the queue.


  • Message messages:
    Let applications exchange messages with another computer.