Ethernet Frame explained.

Actually,Ethernet frames look like

The function of the various parts is as follows:

Preamble/Start of Frame Delimiter, 8 Bytes – Alternate ones and zeros for the preamble, 11010101 for the SFD (Start of Frame Delimiter). This allows for receiver synchronisation and marks the start of frame.

Destination Address, 6 Bytes – The MAC destination address of the frame, usually written in hex, is used to route frames between devices. Some MAC addresses are reserved, or have special functions. For example FF:FF:FF:FF:FF:FF is a broadcast address which would go
to all stations.

Sources Address, 6 Bytes – The MAC address of the sending station, usually written in hex. The source address is usually built into a piece of equipment at manufacture. The first three bytes would identify the manufacturer and the second three bytes would be unique to the equipment. However there are some devices, test equipment for example, in which the address is changeable.

VLAN Tag, 4 Bytes (optional) – The VLAN tag is optional. If present it provides a means of separating data into “virtual” LANs, irrespective of MAC address. It also provides a “priority tag” which can be used to implement quality of service functions.

Length/Type, 2 Bytes – This field is used to give either the length of the frame or the type of data being  carried in the data field. If the length/type value is less than 05DC hex then the value represents the length of the data field. If the value is greater than 0600 hex then it represents the type of protocol in the data field, for example 0800 hex would mean the frame was carrying

1. 809B hex would mean the frame was carrying

AppleTalk.

Data, 46 to 1500 Bytes – The client data to be transported. This would normally include some higher layer protocol, such as IP or AppleTalk.

Frame Check Sequence, 4 Bytes – The check sequence is calculated over the whole frame by the transmitting device. The receiving device will re-calculate the checksum and ensure it matches the one inserted by the transmitter. Most types of Ethernet equipment will drop a frame with an incorrect or missing FCS.