From IP to MAC- What Happens Just Below the Internet Layer

Back to Computer Networks
Arjit Sharma
02 Mins

Let’s say you’re relaxing at home, phone in hand, connected to Wi-Fi. You open the YouTube app and tap on a video. That simple action kicks off a surprisingly complex dance of networking protocols beneath the surface.

Lets checkout what happens below the internet layer by this example.

Link Layer - The Layers Beneath Network

By now you’re probably familiar with the basics:

  • IP: Provides addressing so devices can communicate across networks
  • TCP/UDP: Handles transport of data (reliable or fast, depending on the protocol)
  • HTTP: Tells the server what content you want (like a video)

But these all sit above the link layer, the part responsible for getting data from your phone to the Wi-Fi router and out to the internet.

Local Delivery Needs MAC Addresses

Your phone knows YouTube’s IP address ( 142.250.72.206) from a DNS lookup. But since YouTube isn’t directly reachable, your phone must first send packets to the Wi-Fi router, which acts as a gateway to the internet.

To do that, it needs the router’s MAC address.

  • Your phone’s Wi-Fi chip has its own MAC (a unique hardware ID like AA:BB:CC:DD:EE:FF)
  • So your router is used for local communication only

Your phone asks - “To send my video request to YouTube, I need to pass it to my router first. What’s its MAC address?”

ARP: Finding the Router’s MAC

Here’s how your phone figures it out:

  1. It sends an ARP request over Wi-Fi: “Who has IP 192.168.1.1?” (That’s usually your router’s IP)
  2. Your router replies: “My MAC is AB:CD:EF:12:34:56”
  3. Your phone stores this in its ARP cache and sends the YouTube request framed for that MAC

The Ethernet Frame: Packaging for Local Travel

To get your request to the router, your phone wraps the IP packet in an Ethernet-style frame (yes, even on Wi-Fi):

  • Destination MAC: Your router’s MAC
  • Source MAC: Your phone’s MAC
  • Payload: The IP packet with your HTTP request to YouTube
  • CRC: A code for detecting transmission errors

The Wi-Fi router receives this frame, extracts the IP packet, and forwards it out to YouTube’s servers using its public IP.

Wi-Fi’s Role: Different Transport, Same Concepts

Though Ethernet is typically wired, Wi-Fi works similarly at the link layer just with a different media access technique called CSMA/CA (Collision Avoidance).

Your phone still:

  • Uses MAC addresses
  • Resolves them via ARP
  • Wraps IP packets in frames

It just sends them over radio waves instead of cables.