Software applications on a NetBIOS network locate each other via their NetBIOS names. A NetBIOS name is up to 16 characters long and in Windows, separate from the computer name. Applications on other computers access NetBIOS names over UDP port 137. The Windows Internet Naming Service (WINS) provides name resolution services for NetBIOS.
Two applications start a NetBIOS session when one (the client) sends a command to "Call" another (the server) over TCP port 139 on a remote computer. Both sides issue "Send" and "Receive" commands to deliver messages in both directions. The "Hang-Up" command terminates a NetBIOS session.
NetBIOS also supports connectionless communications via UDP datagrams. Applications listen on UDP port 138 to receive NetBIOS datagrams.
NetBIOS and NetBEUI are separate but related technologies. NetBEUI extends NetBIOS with additional networking capabilities.