Ethernet Frame Structure - buaamer81/IT130-Networking-Wiki GitHub Wiki
📦 Ethernet Frame Structure
📘 Overview
An Ethernet frame is the basic unit of data transmission in an Ethernet network. It wraps the data (payload) with headers and trailers to help with delivery, identification, and error checking.
All Ethernet networks — whether running at 10 Mbps, 100 Mbps, or 10 Gbps — use the same basic frame structure defined by IEEE 802.3.
🧱 Frame Structure Breakdown
An Ethernet frame contains the following fields in order:
| Field | Size | Purpose |
|---|---|---|
| Preamble | 7 bytes | Synchronizes transmission between devices. |
| Start Frame Delimiter (SFD) | 1 byte | Marks the start of the actual frame. |
| Destination MAC | 6 bytes | Address of the receiving device. |
| Source MAC | 6 bytes | Address of the sending device. |
| Type/Length | 2 bytes | Indicates the protocol (e.g., IPv4, IPv6) or frame size. |
| Data/Payload | 46–1500 bytes | Actual data being transmitted. |
| Frame Check Sequence (FCS) | 4 bytes | CRC used for error checking. |
📦 Sample Ethernet Frame Diagram
A simple visual is provided below to illustrate the structure (see PNG below).
📏 Key Facts
- Minimum Ethernet frame size: 64 bytes
- Maximum frame size: 1518 bytes (not including jumbo frames)
- If payload is less than 46 bytes, Ethernet adds padding to reach 64 bytes.
🧠 Ethernet vs. IEEE 802.3 Frames
- Ethernet II uses the Type field to indicate protocol (e.g., IPv4 = 0x0800).
- IEEE 802.3 uses that same field as a length indicator.
- Most modern networks use Ethernet II frame format.