-: The OSI Model.
The Open Systems Interconnection
or OSI Model is used to explain how network communications
occur between a host and a remote device
over a local area network or a LAN.
Now, the OSI Model is very useful to help us
categorize different communication protocols
that are used in networks,
and gives us a common lexicon that we can use
to describe the functions of different devices.
Now, going back to your Network+ studies,
you probably remember the mnemonic
of Please Do Not Throw Sausage Pizza Away.
This represents the seven layers of the OSI Model,
going from the bottom to the top.
This is: Physical, Data Link, Network, Transport,
Session, Presentation, and Application.
The first layer is the physical Layer.
This is the layer that represents
the actual network cables and radio waves
that are used to carry data over a network.
Data carried over the network at the Physical Layer
is known as bits.
And they can be electrical signal or radio wave.
Examples of some of the things that operate
at the Layer One or Physical Layer
are the things like our network cables
whether they're fiber optic, or copper, or coaxial.
It could be radio waves like Wi-Fi and Bluetooth.
It can be a hub or repeater,
which are dumb devices that simply take inputs in
and then repeat them out the other side.
Our second layer is the Data Link Layer.
This is the layer that describes how a connection
is established, maintained, and transferred
over that physical layer.
Addressing here is done using physical addresses,
like MAC addresses.
Now, at this layer, the bits are going to be grouped
into frames and then sent over the network.
Examples of some things that operate at Layer Two
or the Data Link Layer includes things like MAC addresses,
switches, and bridges.
Now, bridges are an earlier device
that have later on evolved into switches.
Switches use MAC addresses as their form
of physical addressing.
This allows a switch to decide
where to send that frame of information
based on the MAC address it's designed to go to.
And so, it's smarter than a hub
because it will decide where that particular
frame goes as opposed to just repeating it out
every single port that it has.
Now, as we move up the ladder, we get to the third layer
which is the Network Layer.
This is the layer where logical addressing
is actually performed.
And this includes things like routing and switching
information between hosts, the network,
and the internetworks.
At this layer, the frames are now taken and grouped up
into packets, so bits became frames, frames become packets.
Now, examples of this include things like
the addresses which are IP addresses.
This allows us to tell where a piece of information,
where a packet is going to be sent over our network.
We also use things like Layer Three switches,
which in addition to using MAC addresses
to decide where things go,
they can use the IP addresses at Layer Three.
And of course routers which are by far
the most common Layer Three device,
because they're used to connect all of our networks
together around the world.
Our fourth layer is the Transport Layer.
This layer manages and ensures transmission
of the packets occur from the host
to the destination it wants.
This uses either a TCP,
known as a Connection Full Protocol
or UDP which is a Connection Less Protocol.
You might remember from Network+,
TCP has that three-way handshake,
and it says, hey I'm ready to send you something,
okay I'm ready to be sent something.
All right, let's start sending it.
And then they send the information.
With UDP, on the other hand,
it's just fire and forget,
we send a bunch of information
and we just hope it gets there.
At this layer, our packets have now
been grouped into segments if we're using TCP
or they're packaged into datagrams
if we're using UDP.
Next, we have the fifth layer which is our Session Layer.
This layer manages the establishment, termination,
and synchronization of a session over the network.
This creates a unique connection over the network
that allows data to find its way from the original host
over to the destination host in a continual sequence.
I like to think about this like making a phone call.
When I pick up the phone and I dial your phone number,
I don't really care which path my voice takes
to get from me to you, which countries it goes through,
or which cities it goes through.
All I care about is that we have a good connection
and that we can talk the entire time.
That is creating a session.
The sixth layer is what's known as the Presentation Layer.
This layer is focused on translating the information
into a format that both the sender and the receiver
are going to understand.
For example, if I send a message to you in text,
you have to know if it's encoded in ASCII or UTF-8.
Depending on which one it is, you're going to read that
information differently, because at heart, all the
sender and receiver are sending are ones and zeroes, right?
And so, those ones and zeroes have to be gone
and translated into a format that we both understand.
Other examples of this might be things like JPGs
or PNG files, or even using encryption.
All of this happens at the Presentation Layer.
It's how are we going to format those ones and zeroes,
and understand how it's going to be displayed
so the end-user can see it.
The seventh and final layer
is known as the Application Layer.
This layer is where the message
is originally created and sent from.
Now, this isn't an application like Word, or PowerPoint,
or Excel but instead, we're talking about
high-level protocols, things like HTTP for web pages,
SMTP for sending email, and FTP for file transfer.
The application layer is where the user
is really starting to interact with the network
by using the OSI Model.
Now, I know I went through all
of this really, really quickly.
And I hope this wasn't brand new information for you.
Again at Security+, they're going to already
assume that you've taken A+ and Network+.
Even if you haven't passed those certification exams,
they're expecting that you've
gone through that curriculum
because it's assumed knowledge for Security+.
With Security+, we're looking at how we can start
securing our networks from attack
and our devices from attack.
And those are all things that we talked about
earlier on in A+ and Network+.
So, if this was all brand new to you,
I do recommend taking a short break here,
going back and watching a Network+ video series
or an A+ video series to make sure
you have that foundational knowledge
as we continue forward
through our Security+ studies together.