NETBIOS
Preface to NetBIOS
Before you begin reading this section, understand that this section was written for the novice to the concept of NetBIOS, but - it also contains information the veteran might find educational. I am prefacing this so that I do not get e-mail like "Why did you start your NetBIOS section off so basic?" - Simple, its written for people that may be coming from an enviroment that does not use NetBIOS, so they would need me to start with basics, thanks. Whats is NetBIOS? NetBIOS (Network Basic Input/Output System) was originally developed by IBM and Sytek as an Application Programming Interface (API) for client software to access LAN resources. Since its creation, NetBIOS has become the basis for many other networking applications. In its strictest sense, NetBIOS is an interface specification for accessing networking services. NetBIOS, a layer of software developed to link a network operating system with specific hardware, was originally designed as THE network controller for IBM's Network LAN. NetBIOS has now been extended to allow programs written using the NetBIOS interface to operate on the IBM token ring architecture. NetBIOS has since been adopted as an industry standard and now, it is common to refer to NetBIOS-compatible LANs. It offers network applications a set of "hooks" to carry out inter-application communication and data transfer. In a basic sense, NetBIOS allows applications to talk to the network. Its intention is to isolate application programs from any type of hardware dependancies. It also spares software developers the task of developing network error recovery and low level message addressing or routing. The use of the NetBIOS interface does alot of this work for them. NetBIOS standardizes the interface between applications and a LANs operating capabilities. With this, it can be specified to which levels of the OSI model the application can write to, making the application transportable to other networks. In a NetBIOS LAN environment, computers are known on the system by a name. Each computer on the network has a permanent name that is programmed in various different ways. These names will be discussed in more detail below. PC's on a NetBIOS LAN communicate either by establishing a session or by using NetBIOS datagram or broadcast methods. Sessions allow for a larger message to be sent and handle error detection and correction. The communication is on a one-to-one basis. Datagram and broadcast methods allow one computer to communicate with several other computers at the same time, but are limited in message size. There is no error detection or correction using these datagram or broadcast methods. However, datagram communication allows for communication without having to establish a session. All communication in these environments are presented to NetBIOS in a format called Network Control Blocks (NCB). The allocation of these blocks in memory is dependant on the user program. These NCB's are divided into fields, these are reserved for input and output respectively. NetBIOS is a very common protocol used in todays environments. NetBIOS is supported on Ethernet, TokenRing, and IBM PC Networks. In its original induction, it was defined as only an interface between the application and the network adapter. Since then, transport like functions have been added to NetBIOS, making it more functional over time. In NetBIOS, connection (TCP) oriented and connectionless (UDP) communication are both supported. It supports both broadcasts and multicasting and supports three distinct services: Naming, Session, and Datagram. NetBIOS Names NetBIOS names are used to identify resources on a network. Applications use these names to start and end sessions. You can configure a single machine with multiple applications, each of which has a unique NetBIOS name. Each PC that supports an application also has a NetBIOS station name that is user defined or that NetBIOS derives by internal means. NetBIOS can consist of up to 16 alphanumeric characters. The combination of characters must be unique within the entire source routing network. Before a PC that uses NetBIOS can fully function on a network, that PC must register their NetBIOS name. When a client becomes active, the client advertises their name. A client is considered to be registered when it can successfully advertise itself without any other client claiming it has the same name. The steps of the registration process is as follows: 1. Upon boot up, the client broadcasts itself and its NetBIOS information anywhere from 6 to 10 to ensure every other client on the network receives the information. 2. If another client on the network already has the name, that NetBIOS client issues its own broadcast to indicate that the name is in use. The client who is trying to register the already in use name, stop all attempts to register that name. 3. If no other client on the network objects to the name registration, the client will finish the registration process. There are two types of names in a NetBIOS enviroment: Unique and Group. A unique name must be unique across the network. A group name does not have to be unique and all processes that have a given group name belong to the group. Each NetBIOS node maintains a table of all names currently owned by that node. The NetBIOS naming convention allows for 16 characters in a NetBIOS name. Microsoft, however, limits these names to 15 characters and uses the 16th character as a NetBIOS suffix. A NetBIOS suffix is used by Microsoft Networking software to indentify the functionality installed or the registered device or service. [QuickNote: SMB and NBT (NetBIOS over TCP/IP work very closely together and both use ports 137, 138, 139. Port 137 is NetBIOS name UDP. Port 138 is NetBIOS datagram UDP. Port 139 is NetBIOS session TCP. For further information on NetBIOS, read the paper at the rhino9 website listed above] The following is a table of NetBIOS suffixes currently used by Microsoft WindowsNT. These suffixes are displayed in hexadecimal format. Name Number Type Usage ========================================================================== <\_MSBROWSE_> 01 G Master Browser |
