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Ping and ICMP

Lesson 13 of 32 in Coddy's The OSI Model course.

Layer 3 has its own control protocol: ICMP. It carries no user data, only signals: "destination unreachable", "TTL expired", and the famous echo request and reply pair that powers ping.

ping sends an ICMP echo request and waits for the reply, proving layer 3 reachability in both directions. It is always the first question to ask a misbehaving network: can I even reach it?

ping -c 1 host sends a single probe, and its exit status tells scripts whether the reply came back.

Challenge

Easy

Ping 127.0.0.1 exactly once, hide all of ping's own output, and print ok if the reply arrived or fail if it did not.

Expected output:

ok

Try it yourself

Terminal

All lessons in The OSI Model

1Why Layers

The Layering Idea Seven Layers Numbers and Names Recap - The Stack

4The Network Layer

IP and TTLPing and ICMPRouting Tables Recap - Layer Three

7Encapsulation

Wrapping Data Headers Everywhere Reading a Capture Recap - Follow the Wrap

2The Physical Layer

Bits on a Wire How Long Will It Take Duplex and Collisions

5Transport: TCP and UDP

Ports The TCP Handshake TCP vs UDP Sockets in Action Recap - Layer Four

8The Journey of a Packet

A Click Happens Crossing the Networks Arriving and Climbing Recap - End to End

3Data Link and MACs

Frames and MACs Reading a MAC ARP Recap - Layer Two

6The Upper Layers

Session and Presentation The Application Layer Watching the Layers Recap - Top of the Stack

Practice on your own: Terminal playground