Ping sweep
We often use the ping command to check if the host is up. If the host answers, we assume that the host is up and we can interact with it.
If the host doesn't answer, then either it's down, our packets are filtered by a firewall, or it doesn't reply to ICMP requests 💀.
If a host doesn't reply to a ping and we know it's up, we may change our strategy, e.g. adapting our commands. For instance, we may use nmap -Pn [...] to disable ICMP checks.