In NetBIOS programming, a network control block (NCB) is a data structure that describes a requested network operation for the Netbios function to interpret and execute. It is an API request record—not a network packet or physical networking component.
What does NCB mean in NetBIOS?
Microsoft describes the NCB as one of the structures used by the NetBIOS interface. An application passes a pointer to an NCB when it calls the Netbios function; the structure specifies the operation and its associated request data. Microsoft summarizes the function’s role this way: “The Netbios function interprets and executes the specified network control block (NCB).” Microsoft Learn’s NetBIOS overview and the Netbios function reference document this API meaning.
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Think of the NCB as control and parameter information for one API request. It is not itself the data packet sent across a network, nor is it a general name for a modern networking architecture.
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How does an NCB request complete?
The completion behavior depends on whether the call is synchronous or asynchronous, as described in Microsoft’s Netbios function reference.
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- Synchronous call: The function returns the NCB return code when the operation completes.
- Asynchronous call: The operation may already be complete when the function returns, or it may still be pending. An NCB can specify an event or a completion routine; the event is signaled when the asynchronous request finishes.
Is “network control block” used in other contexts?
Yes. The phrase is context dependent. Similar names in IBM WSim and IPv4 multicast describe different things, not the NetBIOS request structure.
| Context | What the term represents | Scope |
|---|---|---|
| NetBIOS NCB | A data structure describing an operation submitted to the Netbios function |
One API request; Microsoft Learn |
| IBM WSim NCB | A simulator control block containing information for resources in a simulated network | One simulated network; IBM documentation |
| IPv4 Local Network Control Block | The 224.0.0.0/24 multicast address range for protocol-control traffic that is not forwarded off-link | One multicast address range; RFC 5771 |
RFC 5771 distinguishes that Local Network Control Block from the Internetwork Control Block, whose traffic may be forwarded through the Internet. Neither address block is a software request structure.
Does the NetBIOS NCB matter on current Windows versions?
Microsoft says the Windows Netbios function is provided primarily to support applications written for the NetBIOS interface that need to be ported. The same function reference states that NetBIOS is not supported on Windows Vista, Windows Server 2008, and subsequent Windows versions. For comparable tasks where NetBIOS compatibility is unnecessary, Microsoft points developers to Windows Sockets, mailslots, named pipes, RPC, or distributed COM. This guidance concerns Microsoft’s documented Windows API; it does not establish that every historical NCB implementation behaves identically.
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How can you tell which meaning is intended?
- Terms such as NetBIOS,
Netbiosfunction, or NCB fields indicate the API request structure. - WSim or simulated-network resources indicate IBM’s simulator-specific control block.
- An IPv4 address such as
224.0.0.0/24or discussion of multicast indicates the RFC address range.
When a source says only “network control block,” use its surrounding API, simulator, or multicast context before deciding which definition applies.
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