Ethernet frame format
The Ethernet frame format is a standardized structure used for transmitting data over Ethernet networks. It consists of several fields, each serving a specific purpose. Here’s a breakdown of the Ethernet frame format:
Ethernet Frame Fields:
|
Field
|
Size (Bytes)
|
Description
|
|
Preamble
|
7
|
Synchronization
|
|
Start Frame Delimiter (SFD)
|
1
|
Marks the start of the frame
|
|
Destination MAC Address
|
6
|
MAC address of the destination
|
|
Source MAC Address
|
6
|
MAC address of the source
|
|
EtherType/Length
|
2
|
Protocol type or payload length
|
|
Payload/Data
|
46–1500
|
Actual data being transmitted
|
|
Frame Check Sequence (FCS)
|
4
|
Error detection (CRC)
|
- Preamble (7 bytes)
- Purpose: Synchronizes the receiver’s clock with the sender’s clock.
- Content: Alternating pattern of 1s and 0s (10101010...).
- Start Frame Delimiter (SFD) (1 byte)
- Purpose: Marks the end of the preamble and the start of the actual frame.
- Content: 10101011.
- Destination MAC Address (6 bytes)
- Purpose: Specifies the MAC address of the intended recipient.
- Content: 6-byte (48-bit) hardware address.
- Source MAC Address (6 bytes)
- Purpose: Specifies the MAC address of the sender.
- Content: 6-byte (48-bit) hardware address.
- EtherType/Length (2 bytes)
- Purpose: Indicates the protocol type encapsulated in the payload or the length of the payload.
- Content: If the value is ≤ 1500, it represents the length of the payload. If the value is ≥ 1536, it represents the EtherType (e.g., 0x0800 for IPv4, 0x86DD for IPv6).
- Payload/Data (46–1500 bytes)
- Purpose: Contains the actual data being transmitted.
- Content: The payload can be of variable length, typically ranging from 46 to 1500 bytes. If the data is less than 46 bytes, padding is added to meet the minimum frame size requirement.
- Frame Check Sequence (FCS) (4 bytes)
- Purpose: Used for error detection.
- Content: A 32-bit cyclic redundancy check (CRC) value calculated over the entire frame (excluding the preamble and SFD).
Notes:
- Minimum Frame Size: 64 bytes (including the header and FCS but excluding the preamble and SFD).
- Maximum Frame Size: 1518 bytes (including the header and FCS but excluding the preamble and SFD). For jumbo frames, this can be larger, up to 9000 bytes or more, depending on the network configuration.
This format ensures reliable and efficient data transmission over Ethernet networks.
