Which field in the Ethernet header marks the end of the preamble?

The field in the Ethernet header that marks the end of the preamble is the Start Frame Delimiter (SFD). The SFD is a 1-byte field that follows the 7-byte preamble in an Ethernet frame. The purpose of the preamble is to provide synchronization for the receiving device so that it can read the incoming data correctly. When the SFD is encountered, it signifies that the data portion of the frame is about to begin. This transition from the preamble to the actual data is critical for the correct interpretation of the incoming bits.

Other fields in the Ethernet header, such as the Source Address or Type/Length, have different purposes. The Source Address identifies the sending device, while the Type/Length field indicates either the type of protocol being used or the length of the data contained in the frame. Lastly, the Frame Check Sequence (FCS) is used for error checking at the end of the frame, ensuring data integrity during transmission.