Hack the Route Start free Open app Reconnaissance and exfiltration
Host discovery
Finding which addresses on a network belong to a machine that is up, before looking at any port.
A scanner asks each address whether a machine is there (nmap -sn). On its own network it asks by ARP, which
every machine must answer to be reached at all, so even one that ignores ping shows up. Across a router ARP cannot
reach, so it sends a ping and knocks on a couple of TCP ports (443, 80): any answer, even a reset, proves the host is
up. A host that drops all of them is called down; -Pn skips the question and scans it anyway.