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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 a request record—not a network packet or physical device. The term also has distinct meanings in IBM WSim and IPv4 multicast, so context matters.
What does NCB mean in NetBIOS?
Microsoft describes the Netbios function this way: “The Netbios function interprets and executes the specified network control block (NCB).” The NCB supplies the function with a command and related request data. Microsoft’s NetBIOS overview identifies NCB as one of the interface’s structures: Microsoft Learn: NetBIOS; see also the Netbios function reference.
In practical terms, the NCB is the control-and-parameter record for one API request. It is not itself the data being transmitted over the network, nor a general term for networking hardware.
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How does the Netbios function handle an NCB?
The function receives a pointer to an NCB and processes the operation it describes. Completion behavior depends on whether the call is synchronous or asynchronous, as documented in Microsoft’s Netbios function reference.
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- Synchronous call: The function returns the NCB return code when the call completes.
- Asynchronous call: The request may already be complete when the function returns, or it may still be pending. Completion can be indicated through an event or a completion routine; an event is signaled when the request finishes.
Does “network control block” mean something else?
Yes. The phrase is used in other technical contexts, but those meanings are not interchangeable with the NetBIOS request structure.
| Context | What the block represents | Scope |
|---|---|---|
| NetBIOS NCB | A data structure describing an operation submitted to the Netbios function |
One API request; Microsoft Learn |
| IBM WSim NCB | The primary control block containing information about resources defined 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 | IPv4 multicast addressing; RFC 5771 |
RFC 5771 also distinguishes the Local Network Control Block from the Internetwork Control Block, whose traffic may be forwarded through the Internet. In that RFC, “block” refers to an address range, not a software request structure.
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Is the NetBIOS NCB interface supported on current Windows versions?
Microsoft describes the Windows Netbios function as primarily intended to support applications written for the NetBIOS interface that need to be ported. Its function reference states that NetBIOS is not supported on Windows Vista, Windows Server 2008, and subsequent Windows versions. For similar tasks where NetBIOS compatibility is unnecessary, Microsoft points to Windows Sockets, mailslots, named pipes, RPC, or distributed COM. See the Microsoft Netbios function reference for that API-specific guidance.
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How to identify which meaning applies
- References to NetBIOS, an NCB structure, or the
Netbiosfunction indicate the API request structure. - References to IBM WSim or simulated network resources indicate IBM’s simulation-specific control block.
- References to 224.0.0.0/24, multicast, or protocol-control traffic indicate the IPv4 Local Network Control Block.
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