The Un-Official CCNA guide #15: Fields of Ethernet Frames

Ethernet protocol is defined in IEEE 802.3. It is used to transport information through wires.

Ethernet frames consists of 3 main parts: ethernet header, packet and ethernet trailer.

Ethernet header

There are 5 fields in the Ethernet header

1. Preamble (8 bytes): It is used to synchronise the sender and receiver, so that both send and receive data in the same rate. Consists of a series of 1s and 0s.
2. Start Frame Delimiter (SFD) (1 byte): It is used to signify that the end of preamble. SF field contains 10101011.
3. Destination MAC address (6 bytes): It shows the destination of the frame.
4. Source MAC address (6 bytes): It shows the source of the frame.
5. Length/ ID field (2 bytes): Shows either length of the frame or the type of packet it is carrying.

If this field contains a value 1500 or less, this field contains the length of the frame.

If this field contains a value 1536 or more, this field contains the type of packet it is carrying. (For reference, a value of 0x0800 signifies IPv4 packet while a value of 0x86DD signifies IPv6 packet)

Ethernet trailer

Ethernet trailer contains only 1 field, namely FCS.

1. Frame Check Sequence (FCS) (4 bytes): It is used to check for errors in the frame. FCS uses Cyclic Redundancy Check algorithm to achieve this.