The Ethernet II frame (with the physical-layer preamble/SFD) is built as follows:
| PREAMBLE | START FRAME DELIMITER | DESTINATION MAC | SOURCE MAC | LENGTH OR TYPE | DATA | FRAME CHECK SEQUENCE |
|---|---|---|---|---|---|---|
| 7 bytes | 1 byte | 6 bytes | 6 bytes | 2 bytes | 46–1500 bytes | 4 bytes |
- Preamble: syncs the NIC with the incoming bits
- Start Frame Delimiter (SFD): tells the NIC that the actual frame starts after this field (+ info)
- Destination MAC: self-explanatory
- Source MAC: self-explanatory
- Length or Type: length = how many bytes are in the DATA portion; Type = whether the payload is IPv4 or IPv6
- Data: encapsulated data + protocol information (46–1500 bytes)
- Frame Check Sequence (FCS): used by the receiving device for error-checking – detects errors in the transmission