Nmap

Nmap is a tool for network discovery.

Read the nmap book: https://nmap.org/book/

There are different types of scans, some of them send packets and some of them do not. Sending specific packets to specific ports is how you do host discovery.

Host discovery

• ARP ping

Default on local ethernet. Low weight and not threatening.

ICMP protocol

• ICMP ping

TCP or UDP layer scans to detect if host is up.