"Nmap is a powerful utility for scanning your network and
discovering all kinds of information about who is on it and what
they're doing. You can discover used and unused IP addresses,
hostnames, services, and operating systems, and their versions
– information that can help you monitor who is on your
network, and lead you to unsafe or unauthorized servers.
"Nmap is included in all Linux distros, and the project also
offers binaries for Mac OS X and Windows.
"Nmap is a great tool for finding out who is on your network,
and listing used and unused IP addresses. This command finds all
live hosts in the specified network range and their hostnames:"