IEEE Ethernet Headers and Trailers

I did not explain all the fields inside the headers in this chapter, but several of the fields should look familiar. First, notice that the header begins with a part labeled as 802.3, followed by a part labeled 802.2. Each committee defined different functions, and for those functions, each committee needed a header in which to put some useful information.

The 802.3 part of the header contains several interesting fields. The source and destination Ethernet address fields are in the 802.3 (MAC) header. Because of that, Ethernet addresses are often called MAC addresses. Also, the FCS field is in the 802.3 trailer. At the beginning of the header, the preamble (7 bytes) is shown, with a 1-byte starting delimiter (SD) field. (Remember: The preamble is just a bunch of alternating 1s and 0s, meaning that there's a new frame on the LAN.)

The original DIX Ethernet specifications used an 8-byte preamble field. When the IEEE changed the DIX Ethernet standard, it simply renamed the eighth byte of the preamble to starting delimiter, with no change in the value of the field. The first 8 bytes are still a bunch of alternating 1s and 0s.