Two Standards for Ethernet

Ethernet was originally created in the 1970s by Robert Metcalfe and some others working for Xerox corporation in Palo Alto, California. They got Intel involved, convincing them to put Ethernet logic on a computer chip, making the mass production of Ethernet cards less expensive. They also got Digital Equipment Corporation (Digital), the second-largest computer maker at the time, to support Ethernet. So, the original standard, as defined by those companies, came to be called DIX Ethernet, using the first letters of those three companies in the name. That standard is also known as Ethernet Version 2; the final version created by these three companies was the second version of their Ethernet protocol specifications.

Later, in the 1980s, the IEEE created committees to standardize evolving LAN standards. Basically, the IEEE took work-in-progress on Ethernet and a couple of other types of LANs, made changes, and approved the new specifications as a standard.

Because the IEEE was standardizing several LAN standards at the same time, it noticed that several functions on each type of LAN were similar. So, to create the standards, the IEEE created a committee to define the Ethernet standards and protocols that were unique to Ethernet. It created another committee to establish standards and protocols that are shared between Ethernet and several other types of LANs. The IEEE named the committee for Ethernet-specific standards the 802.3 committee, and it called the committee that defined common LAN features the 802.2 committee. The 802.3 standard is also called Media Access Control (MAC), and the 802.2 standard is also called Logical Link Control (LLC).

In this chapter, you read about several fields that were either in the Ethernet header or trailer. Figure 5-8 shows the header defined by 802.3 and 802.2, as well as the trailer, as defined by 802.3.

Figure 5-8. 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.

You don't really need to memorize where things are inside the header or trailer, but it is useful to look at the format of the headers and trailers and see the fields that the protocols use. The rest of the fields in the figure, although useful, simply didn't make the cut for things to put in this book.