Browsing overview

The information in this topic is provided to explain how browsing a network is done for domains in routed WAN networks that use NetBIOS over TCP/IP (NetBT). Domains used in this type of environment are likely to be controlled by computers running earlier versions of Windows NT operating systems. In this environment, the use of WINS is needed to provide effective browsing of shared resources located in remote domains on other physical networks.

The Computer Browser service is used by Windows-based computers operating on each subnet. Computers designated as browsers maintain the browse lists, which contain all shared resources used on the network. Browsing is required by some network applications provided in earlier versions of Windows such as My Network Places, the NET VIEW command, and Windows NT Explorer.

For example, when a user at a computer running Windows 95 opens My Network Places, the displayed list of domains and computers is generated. This is done by the computer obtaining a copy of the browse list from a browser computer functioning in that role on the same subnet.

Most computers on the network operate as non-browsers but computers running the Computer Browser service can become potential browsers for each subnet. For computers active as browsers on the network and maintaining copies of the browse list, Windows NT assigns special roles.

The following table describes the various roles computers operating this service can perform for the purpose of network browsing:

Browser role	Description
Domain master browser	Used only in domain environments. By default, the primary domain controller (PDC) for a domain operates in this role. Collects and maintains the master browse list of available servers for its domain, as well as any names for other domains and workgroups used in the network. Distributes and synchronizes the master browse list for master browsers on other subnets that have computers belonging to the same domain.
Master browser	Collects and maintains the list of available network servers in its subnet. Fully replicates its listed information with the domain master browser to obtain a complete browse list for the network. Distributes its completed list to backup browsers located on the same subnet.
Backup browser	Receives a copy of the browse list from the master browser for its subnet. Distributes the browse list to other computers upon request.
Potential browser	Under normal conditions, operates similarly to a non-browser. Capable of becoming a backup browser if instructed to by the master browser for the subnet.
Non-browser	Does not maintain a browse list. Can operate as a browse client, requesting browse lists from other computers operating as browsers on the same subnet. Configured so it cannot become a browser.

Under some conditions, such as failure or shutdown of a computer designated a specified browser role, browsers (or potential browsers) may change to a different role of operation. This is typically performed through a process known as a browser election.

Browsing services in earlier versions of Windows operating systems can be understood in terms of three key processes:

Collection of browsing information
Browse lists are made up of computers that share resources through the use of the Server service. Periodically, every computer running this service broadcasts a host announcement message for its configured domain or workgroup name to the local subnet. These announcements are collected and processed on an ongoing basis by the master browser for each subnet.

When the master browser on a subnet receives a host announcement, it compares the name of the sending computer to its current browse list. If the name already appears, it is refreshed in the list. If the name does not appear, it is added to the list.
Distribution of browsing information
Browse lists are distributed to backup browsers by the master browser for the subnet. Periodically, the master browser must broadcast an announcement message for its configured domain or workgroup name to the local subnet. This message confirms the master browser's presence on the network. If the master browser fails to announce itself for a period of time, a browser election can occur.

Once a master browser is either present or (in the event of a failure) replaced, other backup browsers periodically contact it to obtain an updated copy of the browse list that it maintains for the subnet.
Servicing of browse requests from clients
When a browse client computer starts on a subnet, it broadcasts a request to the master browser for a list of the backup browsers on the subnet. The master browser responds to this request and provides the client a list of three backup browsers. The browse client then randomly selects one of the backup browsers from the list and contacts it for a copy of the browse list.

For subsequent browse list requests, the client continues to use the list of backup browsers provided by the master browser during its startup and does not repeat the broadcast. The success of this process depends first upon the client getting a response from its master browser and secondly, its ability to resolve the names of backup browsers it selects from the list.

Example: How browsing works

The following graphic shows an example of these processes for an example domain called MIS.

In this example, the MIS domain has three master browser computers--PDC-MIS, A-MB1, and C-MB1--present on subnets A, B and C. In addition, PDC-MIS, has special significance as the domain master browser for the domain as well as acting as the subnet master browser for subnet B.

As the master browsers on each subnet, these three computers perform the following to create and manage the update and distribution of the MIS domain browse list across all three subnets:

Process browser announcements that are broadcast by any servers on each of their respective local subnets.
Replicate their local subnet browse lists with one another. This eventually builds and replicates a complete browse list for the domain that can be distributed and used on each of the three subnets that have computers participating in the domain.
Replicate their current browse list to backup browsers on each of their respective subnets for fault tolerance and load balancing.
As clients request the browse list, the master browsers for each subnet answer their local clients with the list of backup browsers to use in contacting and obtaining the browse list. Clients then use this list to contact the backup browsers and obtain copies of the actual browse list.

note Note

NetBIOS-based browsing services involve many other details not fully described here. For more in-depth information about browsing, see Updated technical information
For Windows 2000, browsing functionality in many cases can be accomplished by publishing them in the Active Directory through the use of global catalogs. For more information, see Global catalog