NMAP

Nmap is a utility for network exploration or security auditing. It supports ping scanning (determine which hosts are up), many port scanning techniques (determine what services the hosts are offering), version detection (determine what application/service is runing on a port), and TCP/IP fingerprinting (remote host OS or device identification).

Nmap project even offers flexible targets and port specifications, decoy/stealth scanning, SunRPC scanning, and more.

Most of Windows and UNIX platforms are supported in both of the GUI and command line modes as well. Many popular handheld devices are also supported, including the Sharp Zaurus and the iPAQ.
Advantages of Nmap:
• Flexible: Supports dozens of advanced techniques for mapping out networks filled with IP filters, firewalls, routers, and other obstacles.
• PowerfulSodtware: Nmap has been used to scan huge networks of literally hundreds of thousands of machines.
• Portability: Most operating systems are supported.
• Easy to Use: Both traditional command line and graphical (GUI) versions are available to suit your preference.
• Free to Use: The primary goals of the Nmap Project is to help make the Internet a little more secure.
• Well Documented Content: Significant effort has been put into comprehensive and up-to-date man pages, whitepapers, and tutorials.

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