Reachability

Can this host talk to that host on that port? Layered tests: ICMP for IP reachability (ping), TCP for end-to-end open ports (nc -zv), and traceroute / mtr to find where the path breaks when one of the first two fails.

        flowchart TD
    R{reachable?}
    R --> P["ping host<br/>(ICMP echo)"]
    R --> N["nc -zv host port<br/>(TCP handshake)"]

    P -->|yes| HU[host up]
    P -->|no| IP[IP path broken]
    N -->|yes| SO[service open]
    N -->|no| PB[port blocked]

    IP --> TR["traceroute / mtr host<br/>find which hop drops"]
    PB --> RP["try the same on<br/>the relay or its peers"]
    

$ ping -c 3 example.com
$ ping6 -c 3 example.com
$ traceroute example.com
$ mtr example.com
$ nc -zv host 22
$ nc -u -zv host 53