Pen tester tools - NMap

Port scanners allow you to scan a network and collect information quickly. NMap (http://insecure.org/) is probably the most used port scanner. The reason for this is that 1) its free as in beer and 2) its a REALLY good product. That being said, you will find a plethora of guides and forum discussion all over the internet.

Getting to know your network is easy with nmap. Need to find all the hosts on your network:

nmap -sP 192.168.1.0/24

What if an IIS worm is creating crazy traffic on your network, locate all the web servers on the network:

nmap -p80 192.168.1.0/24

I personally like to start with this gem:

nmap -P0 -F 192.168.1.0/24

-P0 disables discovery of hosts by bypassing ping; its great for those computers that drop ICMP packets
-F Fyodor has spent a long time determining what are the most common ports used. These are scanned using the so-called "Fast" mode.

For those who like computer security (AKA hacking), NMap is a great tool to start with.