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NetBIOS over TCP/IP, also called NetBT or NBT, lets applications use NetBIOS network services over a TCP/IP network. Its three services handle name registration and resolution, connectionless datagrams, and connection-oriented sessions. In Microsoft’s familiar Windows mapping, they use UDP ports 137 and 138 and TCP port 139, respectively.
What NetBIOS over TCP/IP means
NetBIOS describes network communication services and an application programming interface (API). NetBIOS over TCP/IP defines how those services work when the underlying network uses TCP/IP. It is a protocol framework for carrying NetBIOS services—not another name for TCP/IP or for SMB.
The foundational specifications are RFC 1001, which describes concepts and methods, and RFC 1002, which gives detailed protocol specifications. Both were published in March 1987. RFC 1001 describes its purpose as defining “a proposed standard protocol to support NetBIOS services in a TCP/IP environment.”
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NetBT’s three service types have different jobs. Microsoft’s Windows documentation associates them with these familiar port mappings:
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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 the standard Windows mappings described by Microsoft, not a guarantee that every implementation or network is configured identically.
Is NetBIOS the same as SMB?
No. SMB is a file-sharing protocol; NetBIOS over TCP/IP is a way to provide NetBIOS services across TCP/IP. In older configurations, SMB traffic can use NetBIOS Session Service over TCP 139. The protocols are related in that setup, but they are distinct.
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What is the difference between ports 139 and 445?
TCP 139 is associated with the NetBIOS Session Service and can carry SMB in legacy configurations. TCP 445 is used by direct-hosted SMB, which does not use NetBIOS. Microsoft documents both paths and notes that Windows 2000 and newer clients can work over TCP 445.
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| SMB path | Transport and port | NetBIOS dependency |
|---|---|---|
| SMB over NetBIOS | TCP 139 for the session service; Microsoft also documents NetBIOS ports UDP 137 and UDP 138. | Uses NetBIOS services; relevant to older systems that rely on SMB over NetBIOS. |
| Direct-hosted SMB | TCP 445 | Does not use NetBIOS. |
Can you disable NetBIOS over TCP/IP?
Whether it is safe to disable depends on what must communicate with your Windows system. Disabling NetBIOS over TCP/IP can prevent communication with older operating systems that rely on SMB over NetBIOS. Systems using direct-hosted SMB over TCP 445 do not depend on NetBIOS for that SMB path. Check the needs of the systems and applications in your environment before changing the setting; compatibility cannot be determined from the port numbers alone.
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