ICMP

Core Idea

ICMP is a network control protocol used for diagnostic and error messages; ping uses ICMP echo requests and echo replies.

Commands To Try

ping -c 3 1.1.1.1
ping -c 3 lf2607.kolamayermakers.org

ping sends ICMP echo requests and reports replies, packet loss, and round-trip time.

How To Read Ping Output

3 packets transmitted, 3 received, 0% packet loss
rtt min/avg/max/mdev = 2.1/2.8/3.4/0.5 ms

• Transmitted: how many requests were sent.
• Received: how many replies returned.
• Packet loss: percentage that did not return.
• Average RTT: rough latency measurement.

What Ping Proves

Ping can prove that ICMP echo replies are coming back from a host or address. It does not prove that HTTP, SSH, DNS, or SMTP works.

Why Ping Can Mislead

• Some networks block ICMP but allow HTTPS.
• Some hosts ignore ping but still serve websites.
• DNS can resolve to an address that does not answer ICMP.
• A load balancer or firewall can respond differently from the backend service.

Common Failures

• Name or service not known: DNS failed before ICMP started.
• 100% packet loss: no replies returned; this may be filtering, routing, or host behavior.
• Destination Host Unreachable: your machine or network stack found no route to the destination.
• Ping works but curl fails: move up the stack and debug the HTTP service.

Proof Check

Run ping -c 3 1.1.1.1, then run curl -I https://example.org. Explain why these test different things.

Docs Pointers

• Run man ping.
• Read Cloudflare: What is ICMP?.
• Read IP Networking for layered network debugging.