ping checks if you can reach a host and how long the host took to reply

$ping health.gov.au

output: ... time=253ms...

Australia is 17,000 km from me. at the speed of light it’s still far!

ping works by sending an ICMP packet and waiting for a reply

ping: to: health.gov.au hello!
health.gov.au: I’m here!

myth: if a host doesn’t reply to ping, that means it’s down

Some hosts never respond to ICMP packets. This is why traceroute shows “…” sometimes.

ping: hello!
host (thinking): not listening!!

traceroute tells you the path a packet takes to get to a destination

me → my ISP → NYC → Sacramento → Australia

example traceroute

$ traceroute health.gov.au
1: 192.168.1.1 3ms ← router
2:...yul.ebox.ca 12 ms ← ISP
...
8: NYC4. ALTER.NET 24 ms ← here the packet crossed the USA! from NYC- Sacramento!
9: SAC1.ALTER.NET 97 ms
...
16: health.gov.au 253ms ← crossing the US takes time

mtr

like traceroute, but nicer output! try it!

last panel

look up how traceroute works (using TTLs!) it’s simple + cool!