Browser System Roles

Computers running Windows 2000, Windows NT 3.1, Microsoft® Windows NT® Advanced Server version 3.1, Microsoft® Windows NT® Workstation version 3.5 or later, Microsoft® Windows NT® Server version 3.5 or later, Windows for Workgroups 3.11, Windows 95, or Windows 98 can be browsers. There are five types of computers in the browser system:

  • Nonbrowsers

  • Potential browsers

  • Backup browsers

  • Master browsers

  • Domain master browsers

Figure I.1 Shows a browser and nonbrowser computers in a subnet.


Figure I.1 Browser and Non-Browser Computers


A non-browser is a computer that has been configured not to maintain a network resource or browse list.

Potential Browser

A potential browser is a computer that is capable of maintaining a network resource browse list and can be elected as a master browser. The potential browser computer can act as a backup browser if instructed to do so by the master browser.

Backup Browser

The backup browser receives a copy of the network resource browse list from the master browser and distributes the list upon request to computers in the domain or workgroup. All Windows 2000 domain controllers are configured as either master or backup browsers.

Computers running Windows 2000 Professional, Windows NT Workstation, Microsoft® Windows® for Workgroups, or Windows 95 can be backup browsers if there are fewer than three Windows 2000 or Windows NT Server computers performing backup browser functions for the domain.

The list of servers is limited in size to 64 kilobyte (KB) on computers running a version of Windows NT earlier than version 4.0, Windows for Workgroups, and Windows 95. This limits the number of computers in a browse list for a single workgroup or domain to between 2,000 and 3,000.


Verbose server comments can significantly reduce the numbers of computers allowable in a browse list since the list size is limited to 64 kilobytes (KB).

Master Browser

The master browser is responsible for collecting the information necessary to create and maintain the browse list. The browse list includes all servers in the domain or workgroup of the master browser and the list of all domains on the network.

Individual servers announce their presence to the master browser by sending a directed datagram called a server announcement to the domain or workgroup master browser. Computers running Windows 2000, Windows NT, Windows for Workgroups, Windows 95, Windows 98, or Microsoft® LAN Manager send server announcements. When the master browser receives a server announcement from a computer, it adds that computer to the browse list.

When a domain spans more than one subnet, the master browser will do the following tasks:

  • Maintain the browse list for the portion of the domain on its subnet.

  • Provide lists of backup browsers on the local subnet of a TCP/IP-based network to computers running Windows 2000, Windows NT, Windows 95, Windows 98, and Windows for Workgroups.

If a TCP/IP-based subnet is comprised of more than one domain, each domain has its own master browser and backup browsers. On networks using NWLink, the IPX/SPX-compatible network protocol, routers are typically configured to forward packets of type 0x14. Since broadcast packets including elections are propagated in this manner, this insures that there is always only one master browser. In contrast, NetBEUI Frame (NBF), which is not designed for a routed network, requires a separate master browser per subnet.

When a computer starts and the value of the MaintainServerList entry in its registry is set to Auto , the master browser must tell that computer whether or not to become a backup browser.

Domain Master Browser

The domain master browser is responsible for collecting announcements for the entire domain, lists from master browsers on other subnets, and for providing a list of domain resources to master browsers. The domain master browser is always the primary domain controller (PDC) of a domain.

The PDC of a domain is given priority in browser elections to ensure that it becomes the master browser. The Windows browser service running on a PDC has the special, additional role of being the domain master browser.

For a domain that uses TCP/IP and spans more than one subnet, each subnet functions as an independent browsing entity with its own master browser and backup browsers. NWLink and NBF transports don't use the domain master browser role because those transports have only a single master browser for the entire network. Browsing across an IP router to other subnets requires at least one browser running Windows 2000, Windows NT, or Microsoft® Windows® for Workgroups 3.11b on the domain for each subnet. A PDC typically functions as the domain master browser on its subnet.

When a domain spans multiple subnets, the master browser of each subnet announces itself as the master browser to the domain master browser, using a directed datagram called a MasterBrowserAnnouncement. The domain master browser then sends a remote NetServerEnum API call to each master browser, to collect the list of servers from each subnet. The domain master browser merges the server list from each subnet master browser with its own server list, forming the browse list for the domain. This process is repeated every 12 minutes to ensure that the domain master browser has a complete browse list of all the servers in the domain.


The Domain Master Browser must be able to resolve the server name of each master browser on a TCP/IP network (using WINS, for example). Each Master Browser must be able to resolve the DOMAIN[1B] name as well as the Primary Domain Controllers machine name.

The master browser on each subnet also sends a remote NetServerEnum API call to the domain master browser to obtain the complete browse list for the domain. This browse list is available to browser clients on the subnet.

A single computer can play multiple browser roles. For example, the master browser might also be the domain master browser.


Windows workgroups cannot span multiple networks. Any Windows workgroup that spans subnets actually functions as two separate workgroups with identical names.