Ethernet Frame

In the previous courses, we learned what a Local Area Network (LAN) is and how devices communicate inside it.
We saw that devices connected to the same LAN exchange data locally through a switch.

Figure 1 - Local Area Network Example

Inside a LAN, communication does not involve a router.
As long as devices are part of the same local network, the switch is responsible for forwarding traffic between them.

This local communication relies on MAC addresses.
Each device has a MAC address, and the switch uses these MAC addresses to decide how data should move inside the LAN.

Communication Inside a LAN Happens at Layer 2

To better understand how this communication works, we need to look at the OSI model.

Communication inside a LAN happens at Layer 2, also known as the Data Link layer.
This is the layer where the switch operates.

Figure 2 – Ethernet operates at the Data Link layer (Layer 2)

Ethernet is the technology used at Layer 2 to enable local communication.
It defines how data is formatted, addressed, and transmitted between devices inside the LAN.

At this point, we know two important things:

• Switches operate at Layer 2

• Switches use MAC addresses to forward traffic inside the LAN

But this raises an important question:

How are MAC addresses actually carried from one device to another?

MAC addresses do not travel alone.
They are transported inside a Layer 2 structure defined by Ethernet.

This structure is called the Ethernet frame.

Inside a Local Area Network, devices need a simple and common way to communicate with each other.

They cannot just send data randomly.
To communicate correctly, all devices must follow the same rules.

These rules define how data is packaged and exchanged inside the LAN.
This is why Ethernet frames exist.

Ethernet Frames: The Common Language of a LAN

As shown in the figure, when PC1 wants to communicate with another device in the LAN, it does not send data directly.

Figure 3 – Devices in a LAN communicate using Ethernet frames

Instead, the data is placed inside an Ethernet frame.

You can think of an Ethernet frame as an envelope:

• the envelope carries the data

• it travels across the LAN

• it is understood by the switch and other devices

Ethernet Frames Are Used for Every LAN Communication

Every device in the LAN uses the same Ethernet frame format.
This common format allows all devices to communicate using the same rules.

Every time devices communicate inside a LAN, an Ethernet frame is created.
There is no local communication without Ethernet frames.

At this stage, remember this simple idea:

Inside a LAN, devices communicate using Ethernet frames.
Ethernet frames provide the structure that makes local communication possible.

Let’s start by looking at an Ethernet frame.

As shown in the figure below, an Ethernet frame is divided into three main parts:

• Header

• Data

• Trailer

Figure 4 – Ethernet frame structure