- How many layers are there in networking?
Do we have a four layer TCP/IP model
or perhaps a five layer TCP/IP model?
Or do we have a seven layer OSI model?
How many layers are there?
Why do we actually care?
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We want standardization.
The reason why I can take a copper Ethernet cable,
such as this, which has a RJ45 connector.
Now we won't get into the details.
It's actually got a different name,
but most of us refer to this as an RJ45 connector.
I wanna be able to take this
and plug it into an HP laptop,
plug it into it to Toshiba laptop,
plug it into an ASUS laptop.
We won't worry about pronunciations for now,
but the moral of the story is I can take this cable
and plug it into different laptops
because this interface has been standardized
and different vendors have agreed to create the equipment
to the specification or to the standard.
That makes life a lot easier.
In the old, old days, vendors just did what they wanted to.
They developed their own protocols
and their own specifications,
which wasn't good for customers.
It was great for the vendor
because if you bought the equipment,
you are locked into that vendor.
We don't want that as a customer.
As a customer, we wanna be able to buy equipment
from different vendors
and they should all be able to work together.
So because of standardization
and because of the industry agreeing on specific models,
this cable can plug into different vendor devices
without any problems.
We take that for granted today,
but it was not always the case.
Now in the previous version of the CCNA,
we were taught the OSI model,
which consists of seven layers.
Starting at the top, it's inverted.
So from layer seven to layer one.
All people sleeping through networking don't pass.
Well, that's not actually what it is.
It's actually application, presentation,
session, transport, network,
data link, and physical layer.
Now that caused a lot of confusion.
A lot of people have debated how relevant
the OSI model is in today's networks.
Have a look here on Twitter.
Rob tweeted, I've got three hours to kill
in an airport lounge, and I'm gonna discuss the OSI model.
There's no such thing.
What they taught you is a lie and they knew it,
but they didn't care.
And that's interesting.
People in the real world are saying
that the OSI model is rubbish.
Other people are saying, well, that's not true.
The OSI model is required.
Now, Ivan is someone that I recommend
that you follow on Twitter
or go to his website and sign up for his free courses.
Ivan is someone who doesn't mess around,
as in he speaks what he thinks.
He will tell you what is true and what is not true.
I highly recommend that you follow Ivan
and perhaps take some of his free courses.
If nothing else, follow him on Twitter
and read his blog posts.
Now, if you're really new to networking,
you may not understand everything that he talks about.
Don't worry, most of us struggle with some of the concepts.
Ivan is really intelligent,
has years and years of experience.
But if you wanna learn from someone
who has got lots of experience
and can help you in your networking career,
follow him on Twitter.
Now basically in his response,
Ivan is saying that we do need the OSI model
and if people had followed the OSI model,
life would be a lot simpler today.
Now I don't wanna muddy the waters
just as you're starting with the OSI model
and the TCP/IP model.
Again, I'll tell you what you need to know for the exam,
but just note that nothing in life
is as simple as it seems sometimes.
There are gray areas.
In the OSI model and the TCP/IP model,
they're taking what's called a protocol stack.
In other words, different protocols that do different things
and they try and put it into neat layers.
Now that's not always clear cut,
and it's not always possible to take protocols
and put them neatly into individual layers
because some developers have programmed the applications
to do things that supposedly they shouldn't be doing.
We are told in the CCNA course
that specific layers should talk to specific layers,
but that doesn't always happen that way.
So moral of the story is for the real world,
things get more complicated
and people get into these rants
or discussions, if you like,
about the OSI model is rubbish,
or we shouldn't be learning the OSI model.
We shouldn't be learning the TCP/IP model.
And then people quote stuff
from various CCNA books showing an original TCP/IP model
and an updated TCP/IP model.
And just to further complicate it,
here's a whole range of different models.
Now the original model comes from RFC 1122.
This is a four layer model.
We have what's called an application layer,
transport layer, internet layer, and link layer.
The Cisco Academy talks about internet model
being application, transport, internet work,
and network interface.
But then on Wikipedia here,
they start quoting different books.
And what you'll notice, we have a five layer TCP/IP model
and then a four plus one layer.
And then right at the end,
we have a seven layer OSI model.
Now the OSI model, once again, is what was taught
in previous versions of the CCNA.
In this version of the CCNA,
they are teaching the so-called five layer TCP/IP model,
which is a combination
of the original RFC 1122 TCP/IP model and the OSI model.
So basically it's a hybrid of multiple models.
Now, the reason we have a model
is we're taking a complex problem
and we're breaking it up into smaller components
or smaller pieces.
This has been done for many, many years.
The Romans had a divide and conquer policy.
So we divide a problem into smaller parts.
And then we conquer smaller parts
rather than trying to do everything in one go.
Models are used in many places.
As an example, if you're building a house,
you typically have a blueprint or a model
of what the house is gonna look like.
So before you even dig the foundations,
a model is created or a blueprint is created
of what the house is gonna look like.
It makes a lot more sense to create a blueprint of a house
and then have specific people work on specific parts
of the building
and do what they good at.
So, as an example, a plumber will work on the plumbing.
An electrician will concentrate on the electricity.
A bricklayer will concentrate on laying the bricks.
But they all work together to create the end result,
which is the house that you want built.
It's gonna be much easier to have a blueprint or a model
that everyone works towards to build something
rather than them just arriving on site
and then saying, let's build this house,
but they don't actually know what the house looks like.
You have an electrician working on the plumbing.
Or a plumber working on bricklaying.
That's not gonna scale very well.
So we have a model of a network
and then we have specific layers or specific components
or subunits, if you like, that people specialize in
and concentrate on that layer.
So manufacturer of fiber cables concentrates
on building this to a specification.
If you're a application developer,
you don't worry so much about the physical layer
or how data is transmitted from,
let's say, the U.S. to Europe,
across the Atlantic using light under this ocean.
You concentrate on building your application.
So we have different layers in the OSI model
and different people concentrate on different layers.
Now, the layers that we as networking people concentrate on
are the lower four layers,
which in the OSI model are called transport,
network, data link and physical.
In the new version of the CCNA,
they are using this hybrid model where they've taken parts
of the OSI model and added it to the TCP/IP model.
So rather than the original model of just a link layer
or network interface layer,
we actually have what's called the data link layer,
which is called layer two.
And the physical layer, which is layer one.
We are counting upwards through the layers
from layer one being the physical layer.
So ignoring all the debates
and all the wars, if you like,
on Twitter about which model is important
and why we care about models,
this is what you need to know.
You need to know both the OSI model and TCP/IP model,
but concentrate on the TCP/IP model.
The OSI model, which consists of these seven layers,
physical layer, data link layer, network layer,
transport layer, session layer,
presentation layer, and application layer,
isn't as important as the TCP/IP hybrid model, if you like.
So a five layer TCP/IP model, which is more real world,
which has a physical layer, data link layer, network layer,
transport layer, and a combined application layer.
But notice, we talk about layer seven applications
because of the history of the OSI model being used.
So notice we have layer one, layer two,
layer three, layer four.
Those are the layers that we concentrate on
as a networking person.
And then we have a combined layer five to layer seven,
called the application layer,
but we still refer to it as application layer.
Now I'm gonna show you this practically
using Packet Tracer, but I want you
to download this yourself and try it.
So don't just follow me, do this yourself
and see if you can answer some of these questions yourself
and see if you can understand the various layers.
In Packet Tracer, they still have the seven layers
of the OSI model, but what you'll notice is layer five
and layer six are basically ignored for the most part.
We concentrate on the lower four layers
and then layer seven, the application layer.
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