This article discusses how resource browsing works across a Wide AreaNetwork (WAN) and how Microsoft's Windows Internet Name Service (WINS)improves cross-subnet browsing.
Browsing with Windows NT version 3.1 (pre-WINS)
In Windows NT version 3.1, browser information transmission relies almostentirely on network broadcasts. In a WAN environment (using a networkprotocol such as TCP\IP) domains are typically separated by routers. Toreduce bandwidth usage, broadcast packets are often filtered and do notpass through routers thus limiting them to the local subnet.
As a result, the Windows NT version 3.1 default B-Node name resolutioncannot resolve names on remote subnets. Each local subnet functions as anindependent browsing entity, with its own Master Browser and BackupBrowsers. Browse elections occur on a per subnet basis.
Windows NT Advanced Server version 3.1 supports cross-subnet browsing. Tobrowse across a WAN, at least one Windows NT Advanced Server is requiredin each subnet. The Domain Master Browser is responsible for maintaining aWAN wide browse list of available servers in the domain and all accessibledomains.
When a domain spans multiple subnets, the Master Browsers for each subnetannounce themselves to the Domain Master Browser using the directeddatagram, "MasterBrowserAnnouncement". The Domain Master Browser thenremotes a "NetServerEnum" call to the Master Browser that announced itselfin order to collect that subnet's list of servers. The Domain MasterBrowser merges the server list from the Master Browser with its own serverlist. This process is done every 15 minutes to guarantee that the DomainMaster Browser has a complete list of servers in the domain.
Now, when a client remotes a "NetServerEnum" call to the Master Browser,the Master Browser will be able to return all of the servers in thedomain, regardless of what subnet they are on.
NOTE: A Windows NT workgroup cannot span multiple subnets. If a Windows NTworkgroup is implemented across two or more subnets, it will function asseparate workgroups.
To ensure each subnet's Master Browser can access every domain's PrimaryDomain Controller (PDC), the Master Browser must maintain a LMHOSTS filecontaining the name of each domain's PDC and Backup Domain Controllers(BDC) with a #DOM extension. To guarantee that each PDC can access thelocal browse list from each subnet's Master Browser, TCP/IP (and other WANprotocols) must cache the client address for a reasonable period of time.
Browsing with Windows NT Server version 3.5 or later with WINS
Windows NT Server versions 3.5 or later include the Windows Internet NameService (WINS). WINS is basically a directory service for NetBIOS names andIP addresses.
The Domain Master Browser registers a unique Browser name of "<DOMAIN>[1B]"with the WINS server. Other Domain Master Browsers will periodically querythe WINS server for a list of domains. They then merge this list into theirlocal domain browse list. They then propagate this list to the MasterBrowser on each subnet via the mechanism mentioned above.
The Domain Master Browser queries the WINS server in a two part process:
- The Browser does a wild card lookup of all domain names ending in [1b].
- Then the Browser does a reverse query for each individual <DOMAIN>[1B] name to learn the unique name of the Domain Master Browser for that domain.
In addition to querying the WINS server, the Domain Master Browserscontinue to collect server lists and domain names in the above mentionedways.
NOTE: Only WINS-aware Domain Master Browsers (those in Windows NT versions3.5 or later) will register their <DOMAIN>[1B] name with the WINS server.Windows NT version 3.1 Domain Master Browsers will not use WINS.
Windows NT Workstation versions 3.5 or later are WINS-Aware
Windows NT Workstation versions 3.5 or later remote the "NetServerEnum"call to a Backup Browser Master for its domain. The list of domainsreturned includes those the Domain Master Browser discovered via the WINSserver. When the Workstation browses a remote domain it will query the WINSserver for the IP address of the computer that registered the <DOMAIN>[1b]name. The Workstation then sends a "GetBackupListReq" to the IP address andpicks one of the machine names from the response and remotes a"NetServerEnum" call to that name to get a list of computers in thatdomain.
Windows NT version 3.1 Clients are Not WINS-Aware
When the Windows NT 3.1 client browses a remote domain, it sends a"GetBackupListReq" to the name <DOMAIN>[1d], not <DOMAIN>[1b]. Becausethere is no Domain Master Browser for this domain in its subnet. Theclient will eventually give up and will perform a "double hop" to its localDomain Browser Master. The browser server will remote the NetServerEnum APIfor the client. How the remote is resolved depends on if the browser serveris WINS aware or not.
Windows for Workgroups version 3.11 Clients with TCP/IP-32 are WINS-Aware
Windows for Workgroups version 3.11 clients can be WINS aware throughTCP/IP-32 for Windows for Workgroups. WINS will help with standardworkstation connections. However, the browser code that they use is notWINS or WAN aware. If the Windows for Workgroups client is in localbrowser domain that has a Windows NT 3.5 or later browser server, theclient will see a list of all remote domains. However, if the clientselects one of these names it will not be able to resolve it. The client isunaware of the special [1b] name.