Ethernet Frame Structure
A frame used in Ethernet (Ethernet II / IEEE 802.3) is made up of several fields. This article explains the role of each field and the frame size.
Description of Each Field
| Field Name | Size | Description |
|---|---|---|
| Preamble | 7 bytes | A bit pattern that repeats 10101010 seven times. Used to synchronize the receiver’s clock with the sender’s. |
| SFD (Start Frame Delimiter) | 1 byte | A fixed pattern of 10101011. Indicates that the frame body (destination MAC address) begins immediately after. |
| Destination MAC Address | 6 bytes | The MAC address of the device the frame is being sent to. Broadcast (FF:FF:FF:FF:FF:FF) and multicast addresses can also be specified. |
| Source MAC Address | 6 bytes | The MAC address of the device that sent the frame. |
| Type/Length (EtherType) | 2 bytes | If the value is 1500 (0x05DC) or less, it represents the data length; if it is 1536 (0x0600) or more, it represents the upper-layer protocol type (EtherType). |
| Data (Payload) | 46–1500 bytes | The data body passed from the upper layer. If it is less than 46 bytes, padding is added to satisfy the minimum frame length. |
| FCS (Frame Check Sequence / CRC) | 4 bytes | A CRC32 checksum calculated from the content between the destination MAC address and the data. Used to detect bit errors during transmission. |
Common EtherType Values
When the value of the Type/Length field is 1536 (0x0600) or greater, that value indicates the upper-layer protocol type (EtherType). Common values are as follows.
| EtherType Value | Protocol |
|---|---|
0x0800 | IPv4 |
0x0806 | ARP |
0x8100 | IEEE 802.1Q VLAN-tagged frame |
0x8847 | MPLS unicast |
0x8848 | MPLS multicast |
0x8863 | PPPoE Discovery Stage |
0x8864 | PPPoE Session Stage |
0x888E | IEEE 802.1X EAPOL |
0x88CC | LLDP |
0x86DD | IPv6 |
EtherType values are managed by the IEEE Registration Authority, and the full list of assigned values is published here.
Frame Size
| Range | Size | Notes |
|---|---|---|
| MAC frame (destination MAC address through FCS) | 64–1518 bytes | The frame size as defined by the Ethernet standard. |
| Total transmitted data (including preamble and SFD) | 72–1526 bytes | The size of the full bit stream actually sent over the cable. |
If the data is less than 46 bytes, padding is inserted to satisfy the minimum frame length of 64 bytes (MAC frame). This minimum size is required for collision detection to function correctly under CSMA/CD.
Also, when an 802.1Q VLAN tag is added, a 4-byte VLAN tag is inserted between the source MAC address and the Type/Length field, extending the maximum frame length to 1522 bytes.