Practice
Nmap ("Network Mapper") is an open-source utility for network exploration and security auditing. It was designed to rapidly scan large networks, although it works fine against single targets too. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) they offer, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. While Nmap is commonly used for security audits, many network and system administrators find it useful for everyday tasks such as monitoring network topology, managing service upgrade schedules, and tracking host or service uptime.
Nmap output is a list of scanned targets, with supplemental information on each depending on the options used. The key information is the "table of important ports". This table contains the port number, protocol, service name, and state. The state can be open, filtered, closed, or unfiltered. Open means that an application on the target machine is ready to accept connections/packets on that port. Filtered means that a firewall, network filter, or some other network obstruction is blocking the port, and Nmap cannot determine whether it is open or closed. Closed ports are not associated with any application, so they could become open at any moment. Ports are classified as unfiltered when they respond to Nmap's probes, but Nmap cannot determine whether they are open or closed. Nmap reports open|filtered and closed|filtered combinations when it cannot determine which of the two states describes the port. This table can also provide software version details if that was requested. When an IP protocol scan is performed (-sO), Nmap provides information about the supported IP protocols instead of open ports.
In addition to the table of important ports, Nmap can provide further information about the targets: resolved DNS names, a guess at the operating system in use, device types, and MAC addresses.
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