Ethernet Frame Format

Ethernet Frames

The original 10 Mbps Ethernet specification defined a simple frame format where each frame is composed of five fields.

0. The Ethernet frame starts with a preamble (not shown in the figure below) that’s used by the physical layer of the receiver to synchronise its clock with the sender’s clock.

1. The first field of the frame is the destination address. As this address is placed at the beginning of the frame, an Ethernet interface can quickly verify whether it’s the frame recipient and if not, cancel the processing of the arriving frame.

2. The second field is the source address. While the destination address can be either a unicast or a multicast/broadcast address, the source address must always be a unicast address.

3. The third field is a 16 bit integer that indicates which type of network layer packet is carried inside the frame. This field is often called the Ether Type. Frequently used EtherType values include: 0x0800 for IPv4, 0x86DD for IPv6, and 0x806 for the Address Resolution Protocol (ARP).

4. The fourth part of the Ethernet frame is the payload. The minimum length of the payload is $46$ bytes to ensure a minimum frame size, including the header of $64$ bytes. The Ethernet payload cannot be longer than 1500 bytes. This size was found reasonable when the first Ethernet specification was written. 1500 bytes was large enough without forcing the network adapters to contain overly large memories.

5. The last field of the Ethernet frame is a 32 bit Cyclical Redundancy Check (CRC). This CRC is able to catch a much larger number of transmission errors than the Internet checksum used by IP, UDP and TCP.

The format of the Ethernet frame is shown below: