Ethernet Source and Destination Addresses

Before sending a frame, Larry adds the Ethernet header and trailer to the data. He puts his own Ethernet address in the source address field of the header and the destination's Ethernet address (Bob again in this case) in the destination address field. After Larry assembles the frame, he can use CSMA/CD logic to determine when he can send the frame.

Also note that Figure 5-6 shows the typical style in which to show a frame, with the header on the left and the trailer on the right. When sending the frame, a NIC sends the bits starting with the header and ending with the trailer, regardless of how it happens to be drawn in a figure.