NetBIOS over TCP/IP—also called NetBT or NBT—is the standard that lets applications use NetBIOS network services across TCP/IP. Its three services handle name resolution, connectionless datagrams, and connection-oriented sessions. In Microsoft’s familiar Windows mapping, they use UDP 137, UDP 138, and TCP 139, respectively. SMB is a separate protocol that can use NetBIOS in older configurations; direct-hosted SMB uses TCP 445.
What NetBIOS over TCP/IP means
NetBIOS describes network communication services and an application programming interface. NetBIOS over TCP/IP defines how those services operate on a TCP/IP network. The foundational standards are RFC 1001 (concepts and methods) and RFC 1002 (detailed specifications), both published in March 1987. RFC 1001 summarizes the purpose as: “This RFC defines a proposed standard protocol to support NetBIOS services in a TCP/IP environment.”
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NetBT and NBT are alternate names for NetBIOS over TCP/IP; they do not refer to SMB itself.
What services it provides and which ports Windows maps to them
The protocol supports three service types. Microsoft’s Windows documentation gives the following familiar port mapping:
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| Service | Purpose | Windows port mapping |
|---|---|---|
| NetBIOS Name Service | Registers and resolves NetBIOS names. | UDP 137 |
| NetBIOS Datagram Service | Sends connectionless datagrams; RFC 1001 characterizes this service as unreliable and non-sequenced. | UDP 138 |
| NetBIOS Session Service | Provides connection-oriented sessions. | TCP 139 |
These are Microsoft’s documented Windows mappings, not a claim that every implementation configures ports identically. Microsoft’s port guidance is available in its service overview and network port requirements.
NetBIOS and SMB are related, but not the same
SMB is a file-sharing protocol. In legacy configurations it can use NetBIOS Session Service, with NetBIOS-related traffic associated with UDP 137, UDP 138, and TCP 139. Microsoft also documents direct-hosted SMB over TCP 445, which does not use NetBIOS. Windows 2000 and newer clients can work over TCP 445, according to Microsoft’s direct-hosted SMB guidance.
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| SMB path | Transport and port | NetBIOS dependence |
|---|---|---|
| SMB over NetBIOS | TCP 139, with NetBIOS name and datagram services using UDP 137 and UDP 138 | Uses NetBIOS services; relevant to legacy configurations. |
| Direct-hosted SMB | TCP 445 | Does not use NetBIOS. |
What disabling NetBIOS over TCP/IP can affect
Whether NetBIOS can be disabled without disrupting communication depends on the systems and services in the environment. Microsoft warns that disabling NetBIOS over TCP/IP can affect communication with older operating systems that rely on SMB over NetBIOS. Systems using direct-hosted SMB over TCP 445 do not use that NetBIOS path; compatibility requirements should be checked before changing a setting.
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