Nmap-scanner

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 of
packet filters/firewalls are in use, and dozens of other
characteristics. 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.

The output from Nmap is a list of scanned targets, with supplemental
information on each depending on the options used. Key among that
information is the "interesting ports table"..  That table lists the
port number and protocol, service name, and state. The state is either
open, filtered, closed, or unfiltered.  Open.  means that an
application on the target machine is listening for connections/packets
on that port.  Filtered.  means that a firewall, filter, or other
network obstacle is blocking the port so that Nmap cannot tell whether
it is open or closed.  Closed.  ports have no application listening on
them, though they could open up at any time. Ports are classified as
unfiltered.  when they are responsive to Nmap's probes, but Nmap cannot
determine whether they are open or closed. Nmap reports the state
combinations open|filtered.  and closed|filtered.  when it cannot
determine which of the two states describe a port. The port table may
also include software version details when version detection has been
requested. When an IP protocol scan is requested (-sO), Nmap provides
information on supported IP protocols rather than listening ports.

In addition to the interesting ports table, Nmap can provide further
information on targets, including reverse DNS names, operating system
guesses, device types, and MAC addresses.