These days those lines are blurred because we have Layer 3 switches but in simple terms a Rada will
transmit packets and a switch will transmit frames.
Now in the real world a lot of networking engineers do not use the OS model in its purest form or original
form.
Network engineers tend to concentrate on the lower four layers of the OS model.
Now that's changing again with the rise of virtualization and technology such as VM way.
But it's still true for many network engineers.
So in its purest form the oversight model has seven layers the application layer at layer seven all
the way down to the physical layer at layer 1.
This model however was not originally developed for TPP.
It was originally developed for protocols such as seal an S and seal and P or connection list mode network
service and connection lists mode network protocol.
Don't worry too much about these protocols unless you using a writing protocol such as ISIS you're probably
not going to encounter these protocols if you're not using ISIS.
TPP had its own model which consisted of the application layer the transport layer Internet layer and
network access layer.
As you can see here the top three layers of the OS on model are grouped into a single layer so application
presentation and session are grouped as a single layer cold application the transport layers equivalent
and the network equivalent is known as the Internet layer.
The data link and physical layers are grouped together into what's called the network access layer in
the TPP model.
Now in the real world we tend to use a hybrid of both of these models so you'll often find that network
engineers group the application presentation and session layer into the application layer but they will
stall referred to the transport network data link and physical layers which are part of the OS model.
They'll even refer to this hybrid model as the over side model sometimes the reason for that is that
most network engineers still tend to spend most of their time looking at the lower four layers of the
OS on model and thus the top three layers are grouped together into a single layer known as the application
layer.
Network engineers tend to concentrate their time and efforts on getting information from one host to
another host rather than being concerned about what happens when the data gets to that host.
The higher layers of the OS on model may still be the realm of the developer or server admin and thus
a lot of network engineers will use the hybrid model but still referred to it as the OS II model.
So just be aware that we have the pure OS on model.
We have the TPP model and then we have this hybrid model.
However because of the growth of VM way a lot of network engineers are having to work with all the layers
of the OS on model.
So this may not be true in your environment.
You may have to troubleshoot applications as well as network infrastructure issues whereas some engineers
never get involved with application troubleshooting.
They spend their time working with routing protocols as an example.
Another important concept to understand is that when host B receives traffic it needs some way of differentiating
protocols used at the higher layers of the OS on model.
So as an example host a may be sending telnet traffic to host B and at the same time maybe sending TFT
P traffic to host B.
How does host B know which of these applications the traffic belongs to.
You could have telnet traffic mingled with TTP traffic so the PCM may receive 10 packets the first three
packets could be TFT P then they may be two packets of telnet traffic and then five packets of TTP.
How does host B know which application to forward the traffic to at higher layers.
Host B may also be running multiple layer 3 protocols so as an example it may be running IP version
4 as well as IP version 6.
How does host B know whether the traffic arriving at layer 2 is IP version 4 or IP version 6.
So when the bets are received at the physical layer of the network interface code of the P.C. it will
read the layer to frame at the data link layer and in that frame if it's using Ethan at 2 which is the
most common type of Ethan it is a field called the type field that denotes or specifies the higher layer
Cole So the network interface card will read the type field the top field informs the network interface
card whether the traffic is IP version 4 or IP version 6.
Or perhaps some other protocols such as OP.
So the top field informs the device which process to send the traffic to at higher layers.
So it could be IP vision for or could be IP version 6 at layer 3.
The protocol number informs the device which protocol is running at least for the two most common protocols
or DCP and UDP.
So at the network layer the protocol field specifies the layer for protocol and at a layer for the port
number specifies the application that's being used which could be telnet.
TFT P H GDP and so forth and so on.
So as an example telnet uses port twenty three TFT P uses port 69 and HDP uses port 80.
But there are many other well-known port numbers.
It's well worth learning some of the well-known port numbers to help you in your networking career.