Nmap Security Scanner is a free and Open Source utility for network exploration or security auditing.

Many systems and network administrators also find it useful for network inventory tasks, managing service upgrade schedules, and monitoring host or service uptime. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. It was designed to scan large networks rapidly but works fine against single hosts.

Nmap Security Scanner runs on all major computer operating systems. Official binary packages are available for Linux, Windows, and Mac OS X. In addition to the classic command-line Nmap executable, the Nmap suite includes an advanced GUI, a results-viewer (Zenmap), a flexible data transfer, redirection, and debugging tool (Ncat), a utility for comparing scan results (Ndiff), and a packet generation and response analysis tool (Nping).

Nmap Security Scanner Features:

Flexible: Supports dozens of advanced techniques for mapping out networks filled with IP filters, firewalls, routers, and other obstacles. This includes many port scanning mechanisms (both TCP & UDP), OS detection, version detection, ping sweeps, and more. See the documentation page.
Powerful: Nmap has been used to scan huge networks of literally hundreds of thousands of machines.
Portable: Most operating systems are supported, including Linux, Microsoft Windows, FreeBSD, OpenBSD, Solaris, IRIX, Mac OS X, HP-UX, NetBSD, Sun OS, Amiga, and more.
Easy: While Nmap offers a rich set of advanced features for power users, you can start as simply as "Nmap -v -A target host." Both traditional command line and graphical (GUI) versions are available to suit your preference. Binaries are available for those who do not wish to compile Nmap from the source.
Free: The Nmap Project's primary goals are to help make the Internet a little more secure and provide administrators/auditors/hackers with an advanced tool for exploring their networks. Nmap is available for free download and comes with full source code that you may modify and redistribute under the license terms.
Well Documented: Significant effort has been put into comprehensive and up-to-date man pages, whitepapers, tutorials, and even a whole book! Find them in multiple languages here.
Supported: While Nmap has no warranty, it is well supported by a vibrant community of developers and users. Most of this interaction occurs on the Nmap mailing lists. Most bug reports and questions should be sent to the nmap-dev list, but only after reading the guidelines. We recommend that all users subscribe to the low-traffic Nmap-hackers announcement list. You can also find Nmap on Facebook and Twitter. For real-time chat, join the #nmap channel on Freenode or EFNet.
Acclaimed: Nmap has won numerous awards, including "Information Security Product of the Year" by Linux Journal, Info World, and Codetalker Digest. It has been featured in hundreds of magazine articles, several movies, dozens of books, and one comic book series. Visit the press page for further details.
Popular: Thousands of people download Nmap every day, and it is included with many operating systems (Redhat Linux, Debian Linux, Gentoo, FreeBSD, OpenBSD, etc.). It is among the top ten (out of 30,000) programs at the Freshmeat.Net repository. This is important because it lends Nmap vibrant development and user support communities.

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