The First Question
When something is unreachable, the first thing to ask is whether the host is even there. The ping tool answers this by sending a small probe and waiting for a reply.
How It Works
Ping rides on a helper protocol rather than on a normal connection.
- It sends an ICMP echo request to the target address.
- A reachable host replies with an ICMP echo reply.
- Ping reports the round trip time and whether any replies were lost.
The Internet Control Message Protocol carries these control and error messages, separate from your TCP or UDP data.
Reading The Output
Each line shows a sequence number and a time in milliseconds. Steady low times mean a healthy path, while missing sequence numbers signal loss. A reply does not prove an application works, only that the host and the network path to it are up.
Note that some hosts and firewalls drop ICMP on purpose, so a silent target is not always down.
Key idea
Ping uses ICMP echo requests and replies to confirm that a host is reachable and to measure round trip time, but a silent host may simply be dropping ICMP rather than being down.