Ethernet Frame Format at Data Link Layer

Ethernet tells what the data should be arranged like what components and compartments it should have. Also header and trailer length 

It is defined by IEEE(Institute of Electrical and Electronics Engineers) 802.3 and divides data into two sublayers. This is the only layer having a sublayer. Works at the data link layer. The order of layers is also very important

Two sublayers :

1. Logical Link Control (LLC)
2. Media Access Control (MAC)

Ethernet frame format  

• Ethernet frame starts with preamble and SFD, they both work at the physical layer
• CRC is the part of the trailer of ethernet frame

Preamble

1. Pattern of alternative 0 and 1 and allow sender and receiver to establish a bit synchronisation. It makes the other device ready

Start of Frame Delimiter (SFD)

1. Has a constant value of 10101011.
2. It indicates that upcoming bits are starting the frame. This is the destination address

Destination Address

1. Contains the MAC address of the machine to which data is to be sent.

Source Address

1. Contains the source MAC address and it's an individual unicast address.
2. The least significant bit of the first byte is always 0 as it's a format.

Length

1. It tells the entire data in the ethernet frame.
2. Hold value from 0 to 65534.
3. But it cannot have a value of more than 1500 as ethernet has some limitations.

Data

1. It is a place where real data is inserted.
2. It is also called Payload.
3. The minimum length is 46 bytes and the maximum size of 1500 bytes.

Cyclic Redundancy Check (CRC) 

1. Part of the trailer and checks the error of data as data may get corrupted due to some noise or other thing which may take 0 to 1 for example
2. This is a 32-bits hash code data
3. It is generated over Destination Address, Source Address, Length and Data field
4. If the checksum received by destination is not the same as the sent checksum value the data received is termed corrupted
5. 64 bytes is the minimum size of the frame
6. 1518 bytes is the maximum size

Also, do not let your curiosity fade away and increase your knowledge on ethical hacking and networking only on Hackers Paradise. Click below to learn more :

If you are curious and want to see the history and evolution of Ethernet which is by the way not so important. You can refer to the Wikipedia article below :