Fields in an Ethernet frame

Networking professionals need to have an idea of the fields within an Ethernet frame, as shown in the following screenshot:

The fields are as follows:

• Preamble: The Preamble is made up of seven bytes and the Start Frame Delimiter (SFD), which is one byte. This is generally used for synchronization between the sender and the receiver devices on a network.
• Source MAC Address: The source MAC address field is made up of six bytes in length which is used to store the sender's MAC address.
• Destination MAC Address: Similarly to the source MAC address field, the destination MAC address field is also six bytes and contains the destination MAC address of the next device along the path to the final destination.
• EtherType: The EtherType field is two bytes, and is used to help identify various protocols which are used by the layers above the Data Link Layer of the OSI reference model.
• Data: The data field ranges between 46 and 1500 bytes per Ethernet frame. This field contains the data from the Application Layer of the protocol suite.
• File Check Sequence (FCS): The FCS is four bytes and is used for error detection in the Ethernet frame. It contains a Cyclic Redundancy Check (CRC), which is a hash value of the frame itself. The sender of the frame would include this hash value before sending to the destination; when the frame is delivered, the recipient device calculates its own version. If the version calculated by the receiver matches the sender's value, there are no issues. However, if the CRC values does not match, the frame is dropped.