Quiz Entry - updated: 2026.07.14
What are the fields of an Ethernet frame, in order?
Preamble + Start Frame Delimiter, Destination MAC, Source MAC, EtherType (Type/Length), Data (payload), and the FCS trailer.
* Fields in order: preamble/SFD (timing, not counted), destination MAC, source MAC, EtherType, payload, and the FCS error-check trailer. *
Ethernet frame fields (in order):
| Field | Purpose |
|---|---|
| Preamble & Start Frame Delimiter (SFD) | Timing/synchronization so the receiver locks onto the bit stream; not counted in the frame size |
| Destination MAC Address | Identifies the receiving NIC (unicast, broadcast, or multicast) |
| Source MAC Address | Identifies the transmitting NIC (always unicast) |
| EtherType (Type/Length) | Indicates the upper-layer (network) protocol carried, e.g., IPv4 (0x0800) or IPv6 (0x86DD) |
| Data (Payload) | The encapsulated Layer 3 packet |
| Frame Check Sequence (FCS) | A CRC trailer used by the receiver to detect transmission errors |
Why it matters: The frame size (min 64 / max 1518 bytes) is measured from the destination MAC field through the FCS — the preamble and SFD are excluded.
Go deeper:
-
Ethernet frame (Wikipedia) — field-by-field breakdown with exact octet sizes for Ethernet II, 802.3 and SNAP. -
Ethernet 802.3 frame structure (Electronics Notes) — header/payload/trailer layout and the inter-frame gap.