[sllug-members]: mac address

[Walt Haas]

Right, there was originally just the destination and source address
followed by a protocol type.  That's called DIX (DEC Intel Xerox)
Ethernet, the way Bob Metcalfe invented it.

Then the IEEE got involved and decided that there should be a bunch of
other fields at the beginning, and the protocol ID field turned into a
length field.  This is called IEEE 802.3 as opposed to Ethernet.
The 802.3 header is supposed to be followed by an IEEE 802.2 packet.

Hardly anybody actually uses the 802.2 packet.  Most protocol stacks
can speak either DIX or 802.3.

You can tell the difference because DIX protocol types are all larger
numbers than the 1536 bytes limit of the original DIX Ethernet frames.
This is used to demultiplex the two types of frames on the same wire.

Novell Netware famously used 802.3 framing without the 802.2 packet
following.  I'm probably responsible for this.  Back in the earliest
days of Novell, three guys from Provo came up to talk to me at the
U of U.  They wanted to know what was the difference between
Ethernet and 802.3.  I told them about the difference between the
protocol and length, but forgot to mention "If you are using 802.3 you
follow the length with an 802.2 header".  So they went back to Provo
and produced the famous Novell packet format with a length field
that didn't have an 802.2 header following, causing grief for years.
Sorry.

A good reference is "Ethernet: The Definitive Guide" published by
O'Reilly.

-- Walt

"Shaun Kruger" <shaun.kruger at gmail.com> wrote:

> > > (original ethernet used this 2 bytes as a
> > > packet length identifier).
> >
> > That is wrong.
> 
> I may have just propagated hearsay that I had come to understand from
> reading about the first experimental ethernet (1972).  I had gathered
> that there wasn't a protocol ID field in 1972 ethernet as the Xerox
> protocol was the only one going over the wire.  The protocol ID was
> needed when it was being standardized so multiple protocols could
> share it.  I'm trying to figure out where I read this.  I may have
> also misunderstood something elsewhere.
> 
> This Wikipedia article is short, but it covers the use of the 2 bytes
> after src and dest mac addresses within a frame.
> http://en.wikipedia.org/wiki/DIX
> 
> It's nice being told you're wrong sometimes.  It makes you check your sources
> .
> 
> Shaun
> 
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