What is NetBIOS?

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Protocol Basics

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Layer:

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Session

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Transport:

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<acronym title="Transmission Control Protocol">TCP</acronym>, <acronym title="User Datagram Protocol">UDP</acronym>

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Port(s):

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137, 138, 139

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RFC(s):

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1001, 1002

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<acronym title="Network Basic Input/Output System">NetBIOS</acronym> is an <acronym title="Application Programming Interface">API</acronym> developed in the early 1980s by IBM to allow applications on separate computers to communicate over a network. Although NetBIOS usually runs over <acronym='transmission control protocol internet protocol'>TCP/IP</acronym> (<acronym title="NetBIOS over TCP/IP">NetBT</acronym>) now, it can also be used over <acronym title="Internetwork Packet Exchange/Sequenced Packet Exchange">IPX/SPX</acronym> and <acronym title="NetBIOS Extended User Interface">NetBEUI</acronym>. NetBIOS provides three services for network communication.

Name Service

Typically, NetBIOS uses broadcasts to register its name on the network. In effect, this service works like a dynamic peer-to-peer <acronym title="Domain Name System">DNS</acronym>. Modern networks implement a centralized name server (Microsoft's <acronym title="Windows Internet Name Service">WINS</acronym> service, for example) to overcome the drawbacks of a broadcast-based name service. NetBIOS over TCP/IP operates the Name Service over 137/UDP.

Session Service

The Session Service provides a method for two devices to establish a connection. Once the connection is established, the Session Service exchanges data between the two end points. NetBIOS over TCP/IP operates the Session Service over 139/TCP.

Datagram Distribution Service

The Datagram Distribution Service handles connectionless communication between devices. This service can send packets to a unique name or broadcast. NetBIOS over TCP/IP operates the Datagram Distribution Service over 138/UDP.

<acronym title="Server Message Block">SMB</acronym> is the network protocol used for file and printer sharing in Windows-based networks. Although they are often confused, it's important to note that SMB and NetBIOS are not the same thing. SMB works at a higher layer than NetBIOS and uses both the Session and Datagram Distribution Services to exchange data.

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