|Ethernet Delivers the Internet
Most home-office or small-office situations bring the Internet into the
local network through an Ethernet connection, leading to a basic router,
interfaced with a DSL modem or a cable modem.
In order to understand the way Ethernet brings us the Internet, we
have to take a little diversion into the concept of messages within
Let me call your attention to a common, ordinary envelope. The outside
of the envelope contains destination address and source address after
the usual pattern that has been established for many, many years. In
this case, the envelope is addressed to the Smith family, of 140 East
Lilac, in Sunol, California. And, of course, the source address is
placed in the upper left-hand corner according to well-established
It turns out that when a postman delivers this envelope to the Smith
family according to this postal address, he’s performing a function
that’s pretty similar to the way Ethernet delivers frames using Ethernet
Our example will get a little more interesting as we open up the
envelope and see what’s inside. Of course, opening the envelope and
looking inside is something that the postman will generally not do. He’s
only concerned with the address on the outside of the envelope.
So, opening up the envelope, we can find out what’s inside. And that
reveals that there’s another package inside the first one; another
envelope inside the envelope, representing a message inside a message.
This time the addressing system is still present, but it’s a little more
intimate. It just says, “To Jessica, from Brian.”
Let’s find out what’s inside by opening up this inner envelope.
We find there a CD with a name on it and a description of its
contents on the outside. Opening up the CD jewel case will reveal that
farther inside is a CD-ROM with additional information on what it
contains. Of course, if we insert that CD into a CD player, we’ll get
some kind of an index of the songs on it.
As it turns out, we know something that the postman does not know, and
that is that Jessica no longer lives here. So, let’s close up the
package and insert it back into the inner envelope with the personal
address identifying Jessica and Brian, and we’ll need to make a new
outer envelope to address it to Jessica’s new, real address. The old
envelope is simply discarded.
....There: a moment later, after addressing the new envelope to
Jessica’s new address, we’ll re-insert the inner package inside the new
envelope, and send it on its way to Jessica’s new address; a message
inside a message.
In the field of computer networking and Internet working, this concept
of placing one package inside another is extended to placing one message
inside another. And it is commonplace. It’s been going on for a long
time, and this practice has become well-known under the name of “Message
Think back now to our first movie in our Ethernet series, the Ethernet
Tutorial. You’ll remember that that tutorial revealed that a lot of the
design for Ethernet is based on the behavior of telegraph operators.
Remember that as telegraph operations matured, the format for messages
became standardized. As telegraph operators sent a message, the first
part of each message was a description of the destination telegraph
office: the destination address to receive the telegram. After the
destination address, the telegram included the source address. In
essence, each telegram started with a preamble that said something like,
“Telegram to Salt Lake City, from Denver”. After those sections of a
telegram identifying the receiving station, and the sending station, the
main body of the text was transmitted.
Take a closer look at this telegram text.
You’ll see it contains a street address. This is a message within the
telegram: a message within the message. And this additional addressing
is necessary because telegraph addresses didn’t go everywhere; every
home did not have a telegraph receiver. A telegraph operator could
address the city and get to an area, but ultimate delivery of the
telegram contents to a person’s home required an additional address. A
boy on a bicycle would sometimes carry a hard-copy printout of a
telegram to a person’s house using the addressing information contained
inside the telegram.
Well, Ethernet frames follow a pattern that’s very similar to telegrams.
Inside the Ethernet frame, after the destination address, and the source
Ethernet address, is the main message field. And that main message field
inside the Ethernet frame can further be broken down into various
sub-fields according to well-developed traditions and standards in the
In fact, so many different people representing different organizations
have had opinions and views and arguments about how to use the Ethernet
message field that influential committees have been formed to create
standards. Among the most important of these little sub-fields is one
that tries to communicate a general description of the type of message
and length of message transported inside this Ethernet frame. The exact
formatting of the bits inside that little field have been a subject of
considerable development and conflicting (some would say "confusing")
But it’s not necessary that we understand those details. What is
important is that we realize and trust that influential committees have
agreed on the format by which information can be exchanged from one
party to another, and that ultimately, our networking equipment can
figure it out.
Probably the single most important message type that can be carried
inside an Ethernet frame is an Internet protocol packet. This is the way
Ethernet delivers the Internet.
I like to think of these committees and the protocols and standards that
they create as if they are forms of the source that I use when I fill
out my income tax reports every year. There’s a place for every bit of
information. And by following the form precisely, other people who’ve
never met me can interpret details about my financial condition without
very much confusion.
By following these well-established forms, or protocol conventions, the
message field inside an Ethernet frame can carry many different kinds of
messages. Our analogy with telegrams is valid here, too: Remember that
the message inside a telegram often contained a postal address. In this
same way, the IP packet, or Internet protocol packet, inside an Ethernet
frame also contains an independent addressing system known as the IP
IP addresses are better-organized for delivery across the world in a
world-wide collection of equipment and networks. It turns out that
Ethernet addresses cannot be used effectively for world-wide
distributions, because they are assigned more-or-less at random. There’s
no rhyme or reason in the local distribution of Ethernet addresses on
your local network. The only requirement of Ethernet addresses is that
they be unique. Imagine how difficult it would be for a world-wide
postal service to address packages and envelopes if everybody’s address
were completely random. Although an individual letter-carrier could
probably memorize the few addresses on his local route, or carry a
simple little map to help him sort out the local addresses, it would be
completely unreasonable to expect world-wide postal delivery systems on
a continental or inter-continental scale to be able to handle completely
random address sorting. And that’s why Ethernet addresses cannot be used
effectively outside your local area network. They just won’t scale to
the world because they are distributed more-or-less at random.
IP addresses, on the other hand, are distributed in a more orderly
fashion by an authority in charge of the organization of the Internet.
And so the patterns in the numbers representing an Internet address help
to locate the networks and sub-networks in the world-wide Internet. This
notion of encapsulation is important to those of us with small networks,
because when we connect our small networks to the Internet, we're
actually merging two networks together. One Ethernet segment is actually
owned and managed by our Internet service provider, and our own Ethernet
is managed independently by our own policies and equipment.
And when an IP packet arrives from the Internet, we must remove it from
the Ethernet frame managed by our ISP network, and repackage it inside a
new Ethernet frame that’s understood by our local networking equipment
for delivery to a computer inside our home or small business.
On the other hand, when we want to send a message out to someone on the
world-wide Internet, our computers create an IP packet, which must be
wrapped in an Ethernet frame for delivery to the Internet. The
world-wide Internet is composed of millions of individual little
networks, and every time an IP packet traverses any of them, it is
encapsulated and then removed from framing such as that which we have
illustrated here with Ethernet.
Ethernet Hubs and Ethernet Switches are not smart enough to do this. In
order to look inside the Ethernet frame, find the internet protocol
packet, interpret its structure, and find and handle the addressing, we
use a smarter box known as a Router. Routers also allow us to share a
single Internet address obtained from our Internet service provider,
with multiple computers inside our home or small office network. You can
learn a lot more about routers elsewhere on AskMisterWizard.com.
This is the text from the movie entitled "Ethernet
Delivers the Internet", published at
(The text also includes a few small, static illustrations from the
movie). You can learn more about the video version and purchase an
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