Nmap - Network scanner

Nmap is a free and open source utility for network discovery and security auditing.
You can find nmap here
For install guide click here
Syntax : nmap scan type options target
For target you can pass hostname , IP address ecc..
-iL filename : Input from list of hosts/networks
-sL: List Scan
-sn: Ping Scan (disable ping scan)
-sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
-sU: UDP Scan
-sO: IP protocol scan
-sY/sZ: SCTP INIT/COOKIE-ECHO scans
-p range port scan : Only scan specified ports
-F: Fast mode
-r: Scan ports consecutively - don't randomize
-O: Enable OS detection
-A: Enable OS detection, version detection, script scanning, and traceroute
-6: Enable IPv6 scanning
-v: Increase verbosity level (use -vv or more for greater effect)
-oN/-oX/-oS/-oG : Output scan in normal, XML, script kiddie, and Grepable format, respectively, to the given filename.
Nmap have a lot others options, you can see that with command : nmap -h

Nmap has also a GUI called zenmap. You can find zenmap here (DON'T RECCOMENDED)

Example :
nmap -O -r -F -sS -vv www.site.com
Os detection : Yes
Version detection, script scanning and traceroute : No
Scan port consecutively : Yes
Fast scan mode : Yes
TCP scan : Yes
UDP scan : No
Target : site.com

nmap -A -F -sU -p 20-25 www.site.com
Os detection : Yes
Version detection, script scanning and traceroute : Yes
Fast scan mode : Yes
TCP scan : Yes
UDP scan : No
Port TCP: 20,21,22,23,24,25
Target : site.com

nmap -A -sU -sS -p T:21,80,139 U:53,111 www.site.com
Os detection : Yes
Version detection, script scanning and traceroute : Yes
Fast scan mode : No
TCP scan : Yes
UDP scan : Yes
Port TCP: 21,80,139 (specified by T:port)
Port UDP: 53,111 (specified by U:port)

If you have a problem or you need some explanations just write under this post!