Ethernet Frames

An Ethernet frame is the unit of data sent over a wired network. Each frame has a fixed structure: a header identifying the sender and receiver, a payload, and a checksum. This guide walks through that structure field by field.

Anatomy of a frame

Fields in transmission order:

• Preamble + SFD (8 bytes): Marks the start of a frame. Not visible to software.
• Destination MAC (6 bytes): Address of the receiving machine.
• Source MAC (6 bytes): Address of the sending machine.
• EtherType (2 bytes): Identifies the payload protocol. Common values: 0x0800 (IPv4), 0x86DD (IPv6), 0x0806 (ARP).
• Payload (46–1500 bytes): Carries the frame’s data. Padded to 46 bytes if shorter.
• FCS (4 bytes): A CRC-32 checksum over the frame. Frames with a mismatched FCS are dropped.

EtherType or length?

The 2 bytes after the MAC addresses depends on its value:

• 0–1500: length in bytes
• 1501–1535: reserved
• 1536+: EtherType

Almost all modern traffic uses an EtherType.

VLAN tags

An optional 4-byte field between the source MAC and EtherType marks the virtual LAN a frame belongs to. Switches use this to separate traffic on shared infrastructure, so tagged frames appear on most enterprise and data centre networks.

Jumbo frames

Some networks support payloads up to around 9000 bytes, well above the standard 1500-byte MTU. The larger size reduces per-byte overhead on high-throughput workloads — common in data centres and storage networks, rare on consumer ones.

Every device along the path must support it, or oversized frames are dropped.

Interactive explorer

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References

• Wikipedia: Ethernet frame
• IANA EtherType registrations