NetBIOS over TCP/IP#

Support for the NetBIOS protocol in Windows when running in a TCP/IP network. NetBIOS over TCP/IP (NBT) supports legacy applications that use the NetBIOS protocol as well as the NetBIOS name server (NBNS), which converts NetBIOS names into IP addresses. NetBIOS names are the names (up to 15-characters long) that users assign to their computers in a DOS/Windows network. See NetBIOS and TCP/IP. NetBIOS over TCP/IP is specified by RFC 1001 and RFC 1002. The Netbt.sys driver is a kernel -mode component that supports the TDI interface. Services such as workstation and server use the TDI interface directly, while traditional NetBIOS applications have their calls mapped to TDI calls through the Netbios.sys driver. Using TDI to make calls to NetBT is a more difficult programming task, but can provide higher performance and freedom from historical NetBIOS limitations.

The Windows 2000 workstation service, server service, browser, messenger, and NetLogon services are all NetBT clients and use TDI to communicate with NetBT. Windows 2000 also includes a NetBIOS emulator. The emulator takes standard NetBIOS requests from NetBIOS applications and translates them to equivalent TDI functions.

Windows 2000 uses NetBIOS over TCP/IP to communicate with prior versions of Windows NT and other clients, such as Windows 95. However, the Windows 2000 redirector and server components now support direct hosting for communicating with other computers running Windows 2000. With direct hosting, NetBIOS is not used for name resolution. DNS is used for name resolution and the Microsoft networking communication is sent directly over TCP without a NetBIOS header. Direct hosting over TCP/IP uses TCP port 445 instead of the NetBIOS session TCP port 139.

By default, both NetBIOS and direct hosting are enabled, and both are tried in parallel when a new connection is established. The first to succeed in connecting is used for any given attempt. NetBIOS over TCP/IP support can be disabled to force all traffic to use TCP/IP direct hosting.