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Crossing the Networks

Lesson 30 of 32 in Coddy's The OSI Model course.

Your packet crosses a dozen routers on its way. Here is the subtle part: at every hop, the frame is torn off and rebuilt, because MAC addresses only mean something on one local segment. The router unwraps to layer 3, picks the next hop, and wraps a fresh frame with new MACs.

The IP addresses stay the same for the whole trip (source: you, destination: the server), while the MAC addresses change at every single hop. Only TTL changes inside the IP header, ticking down by one per router.

One sentence to remember: MACs are per hop, IPs are end to end.

Challenge

Beginner

Print which kind of address changes at every hop, then which kind stays the same end to end. One word per line, uppercase.

Expected output:

MAC
IP

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 TTL Ping 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 HappensCrossing the NetworksArriving 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