How many layers are there in networking.
Do we have a four layer TCPIP model or perhaps a five layer TCPIP model.
Or do we have a seven layer OSI model.
How many layers are there.
Why do we actually care.
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 slipping 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 going to discuss the OSI model.
There's no such thing.
What they taught you was 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 you know take some of these 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 want to 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 have followed the OSI model life would be
a lot simpler today.
Now I don't want to muddy the waters just as you starting with the OSI model and the TCPIP 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 TCPIP 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 TCPIP model and then people quote stuff from various CCNA books showing
an original TCPIP model and an updated TCPIP model.
And just to further complicate it here's a whole range of different models.
Now the original model comes from RFC eleven 22.
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 into network and network interface.
But then on Wikipedia here they start quoting different books and what you'll notice we have a five
layer TCPIP 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 TCPIP model which is a combination of the original RFC 11
22 TCPIP model and the OSI models.
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 going to look like.
So before you even dig the foundations a model is created or a blueprint is created of what the House
is going to 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 are 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 correct to the end result which is the house that you want built.
It's going to 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 going to 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 US 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 TCPIP 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 2 and the physical layer which is layer 1 we are counting
upwards through the layers from layer 1 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 TCPIP model but concentrate on the TCPIP model.
The OSI model which consists of the seven layers physical layer data link layer network layer transport
layer session layer presentation layer and application layer isn't as important as the TCPIP hybrid
model if you like.
So a five layer TCPIP 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 7 applications because of the history of the OSI model being used.
So notice we have layer 1, layer 2, layer 3, layer 4, those are the layers that we concentrate
on as a networking person.
And then we have a combined layer 5 to Layer 7 called the application layer but we still referred 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 5 and layer 6 are basically ignored for for the most
part we concentrate on the lower four layers and then layer 7 the application layer.