The Ethernet Frame is specified by the Ethernet data-link protocol.
It is made up of three parts: an Ethernet header, encapsulated data, and an Ethernet trailer at the end of the frame.
The Different Parts of an Ethernet Frame
- Preamble: Devices use it to synchronize their receiver clocks
- SFD (Start Frame Delimiter): Informs that this is the end of the preamble, and that the next byte corresponds to the Destination field.
- Destination: Receiver’s MAC Address
- Source: Sender’s MAC Address
- Type: Represents the type or the length of the packet, depending on its value:
- A value of 1500 bytes or less => Length
- A value of 1536 bytes or more => Type (usually IPv4 or IPv6)
- Data and Pad: The data packet from the Layer 3. The minimum data size is 46 bytes. If it is less than 46 bytes, the sender adds padding bytes.
- FCS (Frame Check Sequencer): The receiving device uses an algorithm called “CRC” (Cyclic Redundancy Check) to check if there has been errors in the transmission.
Other Things to Know about an Ethernet Frame
Some important numbers:
- Ethernet header + trailer = 18 bytes (destination + source + type + fcs)
- Minimum size of an Ethernet frame (header + data [+ pad] + trailer] = 64 bytes
Resources:
Jeremy’s IT Lab: Free CCNA | Ethernet LAN Switching (Part 1) | Day 5 | CCNA 200-301 Complete Course
Wikipedia: Ethernet Frame