Q119493 - NetBIOS over TCP-IP Name Resolution and WINS

existencetubNetworking and Communications

Oct 26, 2013 (3 years and 10 months ago)

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NetBIOS over TCP/IP Name Resolution
and WINS


The information in this article applies to:



Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0



Microsoft Windows NT Server versions 3.5, 3.51, 4.0



Microsoft Windows 95



Microsoft TCP/IP
-
32 for Windows

for Workgroups, versions 3.11, 3.11a, 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 con
figured 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 rel
iance 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 wor
kstation 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, i
t 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 th
e
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 dir
ected 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 o
ther
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 adv
antage 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 addit
ion, 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 reg
istration
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 configur
ed.