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In 2025, the Ethernet frame structure, defined by IEEE 802.3, is the backbone of wired and wireless networking, enabling reliable data transfer in everything from smart homes to data centers. This guide explores the components of an Ethernet frame, their roles, and their relevance in modern networks like 400GbE and IoT.

Overview of Ethernet Frame Structure

The Ethernet frame encapsulates Layer 3 Protocol Data Units (PDUs) with headers and trailers, ensuring accurate data delivery. Used in Ethernet-II format for TCP/IP networks, the frame structure remains consistent across speeds, from 10 Mbps to 400 Gbps.

Ethernet Frame Components

It ranges from 64 to 1518 bytes (excluding the preamble), with larger jumbo frames (up to 9000 bytes) used in modern networks. Frames smaller than 64 bytes (“runt frames”) or exceeding 1500 bytes without jumbo support are discarded.

Preamble and Start Frame Delimiter (SFD)

• Preamble (7 bytes): Synchronizes sender and receiver, signaling a new frame.
• SFD (1 byte): Marks the frame’s start, ensuring precise timing.
• Role: Prepares devices for frame reception, critical in high-speed networks like 400GbE.

Destination MAC Address

• Size: 6 bytes.
• Function: Identifies the recipient’s MAC address (unicast, multicast, or broadcast).
• Process: Devices compare the frame’s address with their MAC address, accepting matches.

Source MAC Address

• Size: 6 bytes.
• Function: Identifies the sender’s MAC address (always unicast).
• Use: Ensures the recipient knows the frame’s origin.

EtherType

• Size: 2 bytes.
• Function: Specifies the encapsulated protocol (e.g., 0x800 for IPv4, 0x86DD for IPv6, 0x806 for ARP).
• Example: In IoT networks, EtherType identifies lightweight protocols for smart devices.

Data (Payload)

• Size: 46–1500 bytes (padded if smaller to meet 64-byte minimum).
• Function: Carries the Layer 3 PDU (e.g., IP packet).
• Note: Jumbo frames (up to 9000 bytes) are common in data centers for high throughput.

Frame Check Sequence (FCS)

• Size: 4 bytes.
• Function: Detects errors using a CRC algorithm.
• Process: The sender calculates the CRC; the receiver verifies it. Mismatches result in frame rejection.

Field	Size	Function	Example Use Case
Preamble	7 bytes	Synchronization	400GbE timing
SFD	1 byte	Marks frame start	Wi-Fi 7 frame detection
Destination MAC	6 bytes	Identifies recipient	IoT device addressing
Source MAC	6 bytes	Identifies sender	Switch-to-server link
EtherType	2 bytes	Specifies protocol	IPv6 in smart homes
Data (Payload)	46–1500 bytes	Carries higher-layer data	Data center jumbo frames
FCS	4 bytes	Error detection	Ensuring data integrity

VLAN Tagging (IEEE 802.1Q)

• Size: Adds 4 bytes (Tag Protocol Identifier + Tag Control Information).
• Function: Enables virtual LANs for network segmentation.
• Use: Common in enterprise networks for traffic prioritization and security.

Ethernet Frames in 2025

Ethernet frames remain critical in:

Wi-Fi 7: Adapts frame structure for high-throughput wireless.

400GbE: Supports AI and hyperscale data centers with jumbo frames.

IoT: Enables efficient addressing in dense networks.

5G Backhaul: Ensures low-latency frame delivery.