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What’s an Ethernet Frame?

Ethernet frames carry data between network devices using a structured format that includes MAC addresses, a type field, and an error-checking trailer. Understanding this frame structure is foundational to working with wired network communication.

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About this video

Ethernet remains one of the most fundamental and widely deployed networking protocols, forming the backbone of wired communication in homes, offices, and data centers. At the core of how Ethernet operates is the concept of the frame, the structured unit of data that travels between devices on the network. Each device on an Ethernet network has a Network Interface Card with a unique, hardware-assigned MAC address that identifies it at the data link layer. When data is transmitted, it undergoes encapsulation, meaning the original payload is wrapped with additional information to ensure accurate and reliable delivery. The Ethernet frame consists of a header containing a 48-bit destination MAC address and a 48-bit source MAC address, followed by a 16-bit type field that identifies the nature or size of the payload. The payload itself can range from 368 to 12,000 bits depending on the data being carried. A 32-bit Frame Check Sequence is appended as a trailer, giving the receiving device a way to confirm that the frame was not altered or corrupted in transit. When a device receives an Ethernet frame, it reads the destination MAC address first and compares it against its own. If the addresses match, the device processes the rest of the frame. If they do not match, the frame is discarded. This address-based filtering keeps traffic organized and ensures that data reaches only its intended recipient, making the Ethernet frame structure a critical piece of network communication at the hardware level.

What you'll learn

What's covered

Ethernet Frames

Aligned to

Cisco CCNA
1.13 Describe switching concepts
CompTIA Network+
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCST Networking
4.5 Explain basic switching concepts
Cisco CyberOps Associate
4.8 Interpret the fields in protocol headers as related to intrusion analysis

Key terms

Ethernet
A widely used wired networking technology that transmits data over copper cable using electrical signals.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
Network Interface Card
NIC
A hardware component that connects a computer to a network.
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
Payload
The actual data content of a packet or message, excluding headers and metadata.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
Frame Check Sequence
FCS
A cyclic redundancy check value appended to the end of a data link frame that allows the receiver to detect transmission errors. The sending device calculates the FCS from the frame contents, and the receiving device recalculates it; a mismatch means the frame was corrupted and should be discarded.

Transcript

One of the most common protocols there is is Ethernet. And this is an Ethernet cable. In fact, I'm willing to bet that you have Ethernet running somewhere in your home. So let's look at Ethernet, and more specifically at the frames of Ethernet and how it transfers the payload back and forth between devices.

Here's an Ethernet network. In the middle of the Ethernet network, we have a hub. And then each of the devices have a NIC card on them that is an Ethernet NIC card — network interface card — and has a burned-in MAC address to it. So this is what Ethernet is. And what's being sent back and forth, the messages that are being sent back and forth, is called a frame.

Encapsulating an Ethernet Frame

When it comes to encapsulation, we add a header and possibly a trailer to add extra information on how to deliver the data, or the payload. So what does encapsulation look like specifically when we're talking about Ethernet? Well, we have that payload here. We do add a header to it, and we add a trailer in this case.

The header for Ethernet starts out with the MAC address. So that way, when it sends this message, whatever device is receiving it is going to take a look at that destination MAC address and say, does this belong to me? If not, I'm going to get rid of it. If it does belong, if it is the MAC address of the machine that's receiving it, then it'll say, oh, I'm going to process the rest of this. We also have the source MAC address, so that the device knows who this is coming from. So whoever sends this out is going to put their MAC address in the source.

Then we have this type field right here. The type field just essentially identifies what type of Ethernet frame this is. What is the payload going to be? What is the size of all of this? There's essentially two different types that can be found in here, but you can just think of it as identifying the size or the type of this whole Ethernet frame here.

And then we've got this frame check sequence at the end. This is just a double check to make sure that everything is accurate. Once the receiving device receives this, it will use this to verify that nothing's been changed or corrupted along the way.

The Field Sizes

If we were to put actual bits to it, we know that the MAC addresses are 48 bits, so these are each 48 bits. The type field is just 16 bits. The payload could be anywhere from 368 to 12,000 bits. And then the frame check sequence here at the end is 32 bits.

And what is an Ethernet frame? Well, this whole thing is the Ethernet frame. The payload, the header, the trailer — that's what we call the Ethernet frame.

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