Nmap Full Cheat Sheet For Beginners And LEETS

What Is Nmap
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Nmap (“Network Mapper”) is an open source tool for network exploration
       and security auditing. It was designed to rapidly scan large networks,
       although it works fine against single hosts. 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
      ofpacket filters/firewalls are in use, and dozens of othercharacteristics.
       While Nmap is commonly used for security audits, many systems and network
        administrators find it useful for routine tasks such as network inventory,
        managing service upgrade schedules, and monitoring host or service uptime.

Basic Scanning Techniques