💡 Quick recap: Lesson 4 told you the Network wrapper adds a full address, and Data Link adds a local one. Every device actually carries two addresses at once, and they do very different jobs.
1Think of a Car
Every car has two very different ID numbers on it. One is stamped permanently into the metal frame at the factory and never changes, no matter who owns the car or where it moves. The other is a plate issued by whichever state the car is currently registered in — move states, and you get a new one, even though it's the exact same car.
Every device on a network works exactly the same way.
2MAC Address: The Frame-Stamped One
Every network device — your laptop, your phone, your WiFi router — has a MAC address burned in at the factory. It never changes, no matter which network you connect to. It's written in hex (remember Lesson 2?) as six pairs of hex digits separated by colons, like 3E:F2:A0:91:7C:4B.
This is the local address Lesson 4's Data Link wrapper uses — it's how the nearest device on your immediate network recognizes exactly which device it's talking to.
3IP Address: The State-Issued One
An IP address is assigned depending on which network you're currently connected to — your home WiFi hands out one IP, a coffee shop's WiFi hands out a completely different one. It's written as four numbers separated by dots, like 192.168.1.42.
Notice something? Each of those four numbers only ever goes from 0 to 255 — because each one is just a single byte, exactly like your odometer from Lesson 3.
This is the full address Lesson 4's Network wrapper uses — it's how your message finds its way anywhere in the world, not just to the nearest device.
4Try It: Your Device's ID Card
Name a device, generate its two addresses, then watch what happens when it changes networks.
Click that button a few times. The MAC address never moves — it's stamped in. The IP address changes every time, because it depends entirely on which network the device is currently sitting on.
5Why Bother With Two?
Because they answer two completely different questions. The MAC address answers "which exact physical device is this?" The IP address answers "where in the entire world should this message be delivered?" A local address label and a full mailing address are both necessary — one without the other doesn't get a message all the way home.