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I remember back in the
early 1980s, when I first

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started learning networking.

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And I'd studied the
OSI reference model.

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I was pleased as punch when
I would read an article,

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and it would talk
about the application

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layer services of XYZ, or
something at layer three,

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or a new layer two device.

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Because I knew the
functionality of those OSI

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reference model layers.

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And I thought to myself, "OK.

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Great.

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That kind of gives
me an idea of what

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that product or service does.

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But then it also occurred to me,
pretty much at the same time,

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that we would be hard pressed to
find a computer that's actually

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running any type of an
OSI reference protocol.

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Because the OSI reference
model is just that.

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It's a reference model.

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It's not a common, real
protocol that we're using today.

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So I thought to myself,
"Self, what are we using?"

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well, back in the day, we
had lots of other protocols,

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including IPX.

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But we also had this new
and upcoming protocol

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called TCP/IP.

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And TCP/IP was courtesy
of the government.

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The Department of
Defense created it.

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And this TCP/IP IP was actually
a literal protocol stack,

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a suite of protocols
working together

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to provide the functionalities
that were described as part

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of the OSI reference model.

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And so, if we put them
side by side here,

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all the functionality in
the OSI reference model,

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regarding layer
seven, six, and five,

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the application layer
with network services,

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the presentation layer dealing
with encryption and decryption

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and compression, and
the session layer,

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dealing with the establishment,
and the set up of sessions,

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all that functionality can be
found in the TCP/IP Protocol

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Suite, in the one layer
called the application layer.

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And then, layer four, the
transport layer of the OSI

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reference model,
we usually refer

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to that as the
transport layer here,

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as well, on the
TCP/IP protocol suite.

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Also, it's sometimes referred
to as the host to host layer.

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In either case, it's
providing the middle manager

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functionality for reliable
or unreliable delivery,

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based on the
application we're using,

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as well as the potential for
chunking up large messages

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into smaller parcels or pieces.

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But the functionality from the
model and the TCP/IP Protocol

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Suite are the same
at that layer.

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And as we go down to the network
layer of the OSI reference

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model, and the TCP/IP
Protocol Suite is referred

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to as the internet layer.

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However, the responsibilities
and functions

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are the same as
the OSI reference

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models, which include
adding address information,

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like for example, a street
name, and a house number.

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Or, in the case of the
TCP/IP Protocol Suite,

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it would be a
logical IP address,

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which we can think of as a house
number and street name that

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allows a packet to be
delivered over a network

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to the device it was
intended to go to.

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And then as we continue to go
down the TCP/IP protocol stack,

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we have this one section
called Network Access, which

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includes the functionality
of the datalink layer

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and the physical layer of the
OSI reference model, which

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would be our envelope stuffer,
and the correct framing

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for the messages, and the
actual physical transmission

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and sending of the information
over the network itself.

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But there was one more
challenge I discovered.

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And that was, well, if we're
using the TCP/IP protocol

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suite, and I'm
reading this article,

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and it's talking about
a layer two device,

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how does that fit in?

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And so, for the actual
practical purposes for you

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and I, as we work with
TCP/IP networks, which

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is the major protocol in the
world today, when we refer

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to the actual names and
layers, for layer purposes,

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we're going to borrow the
names of physical and datalink

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for layers one and two,
even though we're actually

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using TCP/IP.

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And one of the
benefits of that, it

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allows us to be
very specific when

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we're talking about components.

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For example, if we a
new network adapter,

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and we plug it into
a computer, and it's

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physically connecting us
to the rest of the network,

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we can say that that network
adapter, for that purpose,

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is a layer one device,
a physical layer device.

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Also, for TCP/IP, at layer
three, we refer to layer three

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as the network layer.

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So we're leveraging the name
from the OSI reference model,

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actually renaming
the TCP/IP Protocol

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Suite, the original name,
for that layer three,

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which was Internet,
and simply calling it,

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once again, Network.

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And also, at layer
four, we're hanging

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onto the name of Transport
as the transport layer

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in the TCP/IP protocol suite.

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And then, for
everything above that,

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which would be layers five,
six, and seven from the OSI

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reference model, we just
refer to that one big chunk

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of functions and features
as the application layer.

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So as we discuss the
actual use of a protocol

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stack, and going
back to our analogy

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with our King A and King B,
or a user like Bob, sitting

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in front of a browser, to
access the network services,

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those are being provided by the
application layer of our TCP/IP

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Pericles stack.

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And as that information is
passed down this protocol

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stack, and each of the
responsible parties

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are doing their work, and it's
being sent over the network

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we can use more specific
terms for the data.

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For example, if we have the
middle manager, the transport

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layer, that was currently
working with or looking

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at some data, instead
of calling that data,

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we could refer to
that information

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at that layer as a segment.

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For example, a segment of data.

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And as that information gets
passed down to the mailroom,

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down to the network
layer or three,

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we could refer to the data
at that layer as packets.

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And as the network
layer passes that down

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to the envelope stuffers,
the datalink layer,

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we can refer to the data at that
layer, at layer two, as frames.

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Frames of data.

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And then, as that information
is actually transmitted or sent,

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the actual data
could be referred

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to as individual bits
that are being sent

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over the network at that point.

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So the question might
come up, "Well Keith,

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does that mean that anybody
who says the word packet always

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is talking specifically about
Layer three of the TCP/IP

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protocol suite?"

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And the answer is, no.

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Because some people
use terms loosely.

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We might have to take
things in context.

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But the beautiful thing is this.

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You and I, going forward, if
we say segment, versus packet,

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versus frame, because
we know these terms,

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we can assume that if one of us
says the word segment of data,

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we're referring to the data
here at layer four of the TCP/IP

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Protocol Suite.

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And following that
logic, if you and I were

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to talk about
frames, we would be

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referring to data at layer
two of the protocol stack.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.