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IP and TTL

Lesson 12 of 32 in Coddy's The OSI Model course.

Layer 3 is where IP addresses live and where data can finally leave the local segment. Packets hop from router to router, each router deciding the next step toward the destination.

To stop lost packets from circling forever, every IP packet carries a TTL (Time To Live): a counter, commonly starting at 64, that every router decreases by one. At zero the packet is discarded and an error goes back to the sender.

TTL is also a diagnostic goldmine: tools like traceroute map entire paths just by sending packets with deliberately tiny TTLs.

Challenge

Beginner

A packet leaves your machine with TTL 64 and crosses 3 routers on the way to its destination. Print the TTL the destination sees.

Expected output:

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 ICMP Routing 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