Introduction to NetBIOS

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Introduction to NetBIOS

NetBIOS was developed for IBM in 1983 by Sytek Corporation to allow applications to communicate over a network. NetBIOS defines two entities:

NetBIOS provides commands and support for the following services:

NetBIOS Names

A NetBIOS name is a unique 16-byte address used to identify a NetBIOS resource on the network. This name is either a unique (exclusive) or group (nonexclusive) name. Unique names are typically used to send network communication to a specific process on a computer. Group names are used to send information to multiple computers at one time. An example of a process that uses a NetBIOS name is the File and Printer Sharing for Microsoft Networks service on a computer running Windows 2000. When your computer starts up, this service registers a unique NetBIOS name based on the name of your computer. The exact name used by the service is the 15-character computer name plus a 16th character of 0x20. If the computer name is not 15 characters long, it is padded with spaces up to 15 characters.

NetBIOS name resolution is the process of mapping a computer's NetBIOS name to an IP address. A computer's NetBIOS name must be resolved to an IP address before the IP address can be resolved to a hardware address. Microsoft TCP/IP uses several methods to resolve NetBIOS names; however, the exact mechanism by which NetBIOS names are resolved to IP addresses depends on the NetBIOS node type that is configured for the node. Request for Comments (RFC) 1001, "Protocol Standard for a NetBIOS Service on a TCP/UDP Transport: Concepts and Methods," defines the NetBIOS node types, as listed in Table 1.

Table 1 NetBIOS Node Types

Node Type Description
B-node (broadcast) B-node uses broadcast NetBIOS name queries for name registration and resolution. B-node has two major problems: (1) Broadcasts disturb every node on the network. (2) Routers typically do not forward broadcasts, so only NetBIOS names on the local network can be resolved.
P-node (peer-peer) P-node uses a NetBIOS name server, such as a WINS server, to resolve NetBIOS names. P-node does not use broadcasts; instead, it queries the name server directly.
M-node (mixed) M-node is a combination of B-node and P-node. By default, an M-node functions as a B-node. If an M-node is unable to resolve a name by broadcast, it queries a NetBIOS name server using P-node.
H-node (hybrid) H-node is a combination of P-node and B-node. By default, an H-node functions as a P-node. If an H-node is unable to resolve a name through the NetBIOS name server, it uses a broadcast to resolve the name.


WINS Overview

WINS eliminates the need for broadcasts to resolve computer names to IP addresses and provides a dynamic database that maintains mappings of computer names to IP addresses. WINS is an enhanced NetBIOS name server (NBNS) designed by Microsoft to eliminate broadcast traffic associated with the B-node implementation of NetBT. It is used to register NetBIOS computer names and resolve them to IP addresses for both local and remote hosts.

There are several advantages of using WINS. The primary advantage is that client requests for computer name resolution are sent directly to a WINS server. If the WINS server can resolve the name, it sends the IP address directly to the client. As a result, a broadcast is not needed and network traffic is reduced. However, if the WINS server is unavailable, the WINS client can still use a broadcast in an attempt to resolve the name. Another advantage of using WINS is that the WINS database is updated dynamically, so it is always current. This eliminates the need for an LMHOSTS file. In addition, WINS provides network and interdomain browsing capabilities.

Before two NetBIOS-based hosts can communicate, the destination NetBIOS name must be resolved to an IP address. This is necessary because TCP/IP requires an IP address rather than a NetBIOS computer name to communicate. Resolution uses the following process:

  1. In a WINS environment, each time a WINS client starts, it registers its NetBIOS name/IP address mapping with a configured WINS server.
  2. When a WINS client initiates a command to communicate with another host, the name query request is sent directly to the WINS server instead of being broadcast on the local network.
  3. If the WINS server finds a NetBIOS name/IP address mapping for the destination host in this database, it returns the destination host's IP address to the WINS client. Because the WINS database obtains NetBIOS name/IP address mappings dynamically, it is always current.