NetBIOS Over TCP/IP (NBT)

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NetBIOS Over TCP/IP (NBT)

When the OSI networking model, of which NetBIOS is a part, is compared to the TCP/IP networking model, it is essential to understand that the first layer in the TCP/IP stack is also the application layer. However, it encompasses the functions of the top three layers of the OSI model (see Figure 7.1). Because this is also where NetBIOS resides in the OSI model, some method is required to map the NetBIOS functions to the TCP/IP functions. Sitting between the TCP/IP application layer and the Transport layer is the Winsock interface. Winsock provides end points for communications. For example, to connect to a Web site, you call an IP address, protocol, and port number (for instance, 199.45.92.97:TCP:80 is a Web page address). The port number is the Winsock port on which the requested service lives.

Therefore, for NetBIOS to function over TCP/IP, NetBIOS needs to use ports. Three ports have been assigned to NetBIOS on which to send information and listen for incoming traffic. Table 7.1 lists the three NetBIOS ports that are used.

As shown in Table 7.1, the ports for the NetBIOS Name Service and the NetBIOS Datagram Service use UDP (User Datagram Protocol). This means that no session is required to transmit information. NetBIOS is based heavily on broadcasts. This system dates to the time when NetBIOS was developed; the networks were smaller and typically only one segment, so broadcasts worked well. As this discussion on NetBIOS continues, you will discover that using broadcasts now represents a major problem.

Most routers are configured not to forward broadcasts on ports 137 and 138 (although they can pass directed transmissions). NetBIOS uses broadcasts for many functions and the amount of broadcast traffic can congest routers. This can cause problems because functions such as domain validation and browser services require the use of broadcasts. These problems are addressed in Chapter 10, “IP Internetwork Browsing and Domain Functions.”

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