NetBIOS over TCP/IP Name Resolution and WINS

• Microsoft Windows NT Workstation version 3.5
• Microsoft Windows NT Server version 3.5
• Microsoft Windows NT Workstation versions 3.5 and 3.51
• Microsoft Windows NT Server versions 3.5 and 3.51
• Microsoft Windows NT Workstation version 4.0
• Microsoft Windows NT Server version 4.0
• Microsoft Windows 95
• Microsoft TCP/IP-32 for Windows for Workgroups, versions 3.11, 3.11a, and 3.11b

SUMMARY

NetBIOS over TCP/IP is the network component that performs computer name to IP address mapping, name resolution (NETBT.SYS in Windows NT and VNBT.VXD in Windows for Worgroups and Windows 95). There are currently four NetBIOS over TCP/IP name resolution methods: b-node, p-node, m-node and h-node.

MORE INFORMATION

B-Node

When using b-node, broadcasts are used for both name registration and name resolution. On a TCP/IP network, if the IP router is not configured to forward the name registration and name query broadcasts, systems on different subnets will not be able to see each other since they will not receive the broadcasts. B-node name resolution is not the best option on larger networks because its reliance on broadcasts can load the network with broadcasts.

Microsoft Modified B-Node

The TCP/IP used in Microsoft Windows NT uses a modified version of b-node name resolution. Microsoft modified b-node name resolution works in the following manner:

• The workstation first checks the LMHOSTS cache and, if it finds the NetBIOS name, returns the IP address.
• Next, the workstation tries broadcasting to resolve the name (this works in the same manner as b-node resolution) and, if the destination system is active, it returns its IP address.
• Finally, the workstation (if it is a Windows NT system) will check the LMHOSTS file in the \<winnt_root>\system32\drivers\etc directory

P-Node (or Point to Point Node)

When using p-node name resolution, broadcasts are NOT used for name registration or name resolution. Instead, all systems register themselves with a NetBIOS Name Server (NBNS) upon start up. The NBNS is responsible for mapping computer names to IP addresses and making sure that no duplicate names are registered on the network. All systems must know the IP address of the NBNS, which is equivalent to a WINS Server. If the systems are not configured with the correct IP address for the NBNS, p-node name resolution will not work.

The p-node name resolution method uses directed User Datagram Protocol (UDP) datagrams and TCP sessions for its communication to and from the NBNS.

The main drawback of p-node name resolution is that if the NBNS cannot be accessed, there will be no way to resolve names and thus no way to access other systems on the network.

M-Node (or Mixed Node)

M-node uses a combination of b-node and p-node for name resolution. This method first uses b-node and then p-node, which in theory should increase local area network (LAN) performance. M-Node has the advantage over p-node in that if the NBNS is unavailable, systems on the local subnet can still be accessed through b-node resolution.

M-node is typically not the best choice for larger networks because it uses b-node and thus results in broadcasts. However, when you have a large network that consists of smaller subnetworks connected via slow Wide Area Network (WAN) links, M-node is a preferred method since it will reduce the amount of communication across the slow links.

H-Node (or Hybrid node)

H-node name resolution, which is currently in RFC draft form, also uses both b-node and p-node, however it only uses b-node as a last resort. When configured to use h-node, a system will always first try to use p-node and then use b-node ONLY if p-node fails. In addition, a system can be configured to use the LMHOSTS file after p-node fails and before trying b-node.

H-node resolution does not require successful p-node registration for a system to initialize, however the system will use strictly b-node until p-node registration succeeds. If the NBNS is unavailable and the system resorts to using b-node resolution, it will continue to attempt to contact the NBNS so that it can return to using p-node if the NBNS becomes available.

How WINS Works

By default, when a system is configured to use WINS for its name resolution, it adheres to h-node for name registration. For name resolution, it will also adhere to h-node but with a few differences. It will:

• Check to see if it is the local machine name.
• Check its cache of remote names. Any name that is resolved is placed in a cache where it will remain for 10 minutes.
• Try the WINS Server.
• Try broadcasting.
• Check the LMHOSTS file, if the system is configured to use the LMHOSTS file.
• Try the HOSTS file and then a DNS, if so configured.