Source MAC Address - Ethernet Header

Definition

• The Source MAC Address is a 6-byte (48-bit) field that identifies which device sent the Ethernet frame.
• Its role is to let receiving devices and switches know where the frame came from, so they can learn network paths and send replies correctly.

What Problem It Solves

When a device receives a frame, it needs to know who sent it. Switches also use the Source MAC Address to build and update their MAC address table, allowing future frames to be forwarded efficiently instead of being flooded.

What Happens If Not Used

The receiver would not know where to send a reply, and switches could not learn which devices are connected to which ports. This would cause frequent flooding and poor network performance.

Easy Wording

The Source MAC Address is the return address on the Ethernet package.

Layman Example

You receive a letter with both the recipient's address and the sender's address. If you want to reply, you use the sender's address.

Technical Example

• Your browser requests a webpage.
• Your PC creates an Ethernet frame.
• Destination MAC = your router's MAC.
• Source MAC = your PC's NIC MAC.
• The switch receives the frame and reads the Source MAC.
• It records: "This MAC is connected to Port 5."
• When another device later sends data to your PC, the switch already knows which port to use.

Limitation: The Source MAC Address identifies who sent the frame, but it does not indicate what kind of data is inside. Type/Length, tells the receiver how to interpret the payload.