NetBIOS over TCP/IP (NBT) is the session-layer network service that performs name-to-IP address mapping for name resolution. This section describes the modes of NBT, as defined in RFCs 1001 and 1002 to specify how NetBIOS should be implemented over TCP/IP.
The modes of NBT define how network resources are identified and accessed. The two most important aspects of the related naming activities are registration and resolution. Registration is the process used to acquire a unique name for each node (computer system) on the network. A computer typically registers itself when it starts. Resolution is the process used to determine the specific address for a computer name.
The NBT modes include the following:
If WINS servers are specified by either a DHCP server or the TCP/IP configuration specified in the Network option of Control Panel, Windows NT 3.5 defaults to h-node. Otherwise, the default node type is b-node, unless another node type has been set as an option by the DHCP server.
For DHCP users on a Windows NT network, the node type is assigned by the DHCP server. A DHCP client computer can have only one NetBIOS node type, no matter how many adapter cards it has. On a multihomed computer with access to more than one network, the node type must be the same on both networks. When WINS servers are in place on the network, NBT resolves names on a client computer by communicating with the WINS server. If you want to configure a multihomed computer with some network adapter cards using b-node and some using h-node, configure WINS server addresses for the adapter cards that are to run in h-mode. The presence of a WINS address on an adapter card effectively overrides the b-node setting.
When WINS servers are not in place, NBT uses b-node broadcasts to resolve names. NBT in Windows NT can also use LMHOSTS files and DNS for name resolution, depending on how TCP/IP is configured on a particular computer. In Windows NT 3.5, the NETBT.SYS module provides the NBT functionality that supports name registration and resolution modes.
Windows NT version 3.5 supports all of the NBT modes described in the following sections. NBT is also used with the LAN Manager 2.x Server message protocol.
The b-node mode uses broadcasts for name registration and resolution. That is, if NT_PC1 wants to communicate with NT_PC2, it broadcasts to all machines that it is looking for NT_PC2, and then it waits a specified time for NT_PC2 to respond. B-node has two major problems:
The p-node mode addresses the issues that b-node does not solve. In a p-node environment, computers neither create nor respond to broadcasts. All computers register themselves with the WINS server, which is a NetBIOS Name Server (NBNS) with enhancements. The WINS server is responsible for knowing computer names and addresses and for ensuring no duplicate names exist on the network. All computers must be configured to know the address of the WINS server.
In this environment, when NT_PC1 wants to communicate with NT_PC2, it queries the WINS server for the address of NT_PC2. When NT_PC1 gets the appropriate address from the WINS server, it goes directly to NT_PC2 without broadcasting. Because the name queries go directly to the WINS server, p-node avoids loading the network with broadcasts. Because broadcasts are not used and because the address is received directly, computers can span routers.
The most significant problems with p-node are the following:
The m-node mode was created primarily to solve the problems associated with b-node and p-node. This mode uses a combination of b-node and p-node. In an m-node environment, a computer first attempts registration and resolution using b-node. If that is successful, it then switches to the p-node. Because this uses b-node first, it does not solve the problem of generating broadcast traffic on the network. However, m-node can cross routers. Also, because b-node is always tried first, computers on the same side of a router continue to operate as usual if the WINS server is down.
M-node uses broadcasts for performance optimization, because in most environments local resources are used more frequently than remote resources. Also, in a Windows NT network, m-node can cause problems with NetLogon in routed environments.
The h-node mode, which is currently in RFC draft form, is also a combination of b-node and p-node that uses broadcasts as a last effort. Because p-node is used first, no broadcasts are generated if the WINS server is running, and computers can span routers. If the WINS server is down, b-node is used, so computers on the same side of a router continue to operate as usual.
The h-node mode does more than change the order for using b-node and p-node. If the WINS server is down so that local broadcasts (b-node) must be used, the computer continues to poll the WINS server. As soon as the WINS server can be reached again, the system switches back to p-node. Also, optionally on a Windows network, h-node can be configured to use the LMHOSTS file after broadcast name resolution fails.
The h-node mode solves the most significant problems associated with broadcasts and operating in a routed environment. For Microsoft TCP/IP users who configure TCP/IP manually, h-node is used by default, unless the user does not specify addresses for WINS servers when configuring TCP/IP.
Another variation is also used in Microsoft networks to span routers without a WINS server and p-node mode. In this mode, b-node uses a list of computers and addresses stored in an LMHOSTS file. If a b-node attempt fails, the system looks in LMHOSTS to find a name and then uses the associated address to cross the router. However, each computer must have this list, which creates an administrative burden in maintaining and distributing the list. Both Windows for Workgroups 3.11 and LAN Manager 2.x used such a modified b-node system. Windows NT uses this method if WINS servers are not used on the network. In Windows NT, some extensions have been added to this file to make it easier to manage (as described in Chapter 15, "Setting Up LMHOSTS"), but modified b-node is not an ideal solution.
Some sites might need to use both b-node and p-node modes at the same site. Although this configuration can work, administrators must exercise extreme caution in doing so, using it only for transition situations. Because p-node hosts disregard broadcasts and b-node hosts rely on broadcasts for name resolution, the two hosts can potentially be configured with the same NetBIOS name, leading to unpredictable results. Notice that if a computer configured to use b-node has a static mapping in the WINS database, a computer configured to use p-node cannot use the same computer name.
Windows NT computers can also be configured as WINS proxy agents to help the transition to using WINS. For more details, see the next section.