How to Scan for Computers that Use NetBIOS

Your tool of choice is a port scanner. Any computer that is running something on port 139 is likely (but not certain) to be using NetBIOS. Most crackers use nmap to port scan. This tool runs on Unix/Linux type computers. You can get it at

Here's an example of an nmap scan of Oldguy:

test-box:/home/cmeinel # nmap -sTU

Starting nmap V. 3.00 ( )
Interesting ports on (
(The 3060 ports scanned but not shown below are in state: closed)
Port State Service
21/tcp open ftp
70/tcp open gopher
80/tcp open http
135/tcp open loc-srv
135/udp open loc-srv
137/udp open netbios-ns
138/udp open netbios-dgm
139/tcp open netbios-ssn
500/udp open isakmp

Nmap run completed -- 1 IP address (1 host up) scanned in 8 seconds

As you can see from this scan, three ports are identified with NetBIOS. This tells us that we could set nmap to scan a large number of Internet addresses, only looking for port 139 on each. To learn how to set up nmap to run this way, in your Unix or Linux shell give the command "man nmap".

You can get punched in the nose warning: if you use a port scanner against networks that haven't given you permission to scan, you will be waving a red flag that says "Whaddaya wanna bet I'm a computer criminal?" You can't get arrested for merely port scanning, but people who don't like being scanned might get you kicked off your Internet service provider.

You can get really, big time, punched in the nose warning: If you visit the same computer or LAN really often to see what's new and to try different things, even if you don't break the law you'd better be doing it with the permission of the owner. Otherwise you may make enemies who might crash or destroy your operating system. And that is only what they may do when feeling mellow. After a night of hard drinking - well, you don't want to find out.

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