All right.
So this whole time we've been talking we've been talking about networking and I've been throwing terms
at you and I every time I throw a term at you I try to use the respective layer for it.
So you've heard me say layer to layer three layer four.
And those all correspond with what is called the ISI model.
Now if you're ever in a network interview or if you're ever talking to somebody who has experience in
networking or even if you're on the Help Desk taking tickets knowing the ISI model is incredibly helpful.
And people will just throw layers at you especially the people who have been in the field for quite
some time might just say layer two instead of a switch or they might say layer three instead of a router.
So I'm going to discuss the ISI model really quickly give you a pneumonic on how to remember it and
just talk about some of the concepts within it and how to troubleshoot down it as well.
So I pick this up from Keith Barker a long time ago great trainer by the way.
And this is the mnemonic.
SO WE'RE GOING TO GO P. D and T S P A and this stands in my head for.
Please do not throw sausage pizza away again that is please do not throw sausage pizza away.
So I'm going to put numbers corresponding to the layers in front of it here and we're gonna go ahead
and type these out one by one.
So on the first layer here we've got what is called the physical layer and you can think of your physical
layer as like a data cables or like your cat six cables.
Stuff like that something you might you might plug in.
Right.
That is the physical layer.
And we've already talked about layer to quite a bit Layer two is the data layer and that is our switching
right.
And also our mac addresses going down the list.
We've got the network layer which is IP addresses also routing the fourth layer is the transport layer
which is DCP UDP which we have talked about as well.
And the last few.
So the session layer we've got the session layer which is just session management.
You don't really have to worry too much about this one.
Six is the presentation layer.
Now this should be familiar to you because think about WNBA j peg movie files.
That's what your presentation layer and so media.
And then lastly we've got the application layer which is like a DP S.M. T.P. your applications that
you utilize.
Right.
So we've got this laid out here.
And you might be asking why is this important.
Well again when we say something like my home router is a layer two three.
So that means it does it does switching and it does routing.
Right.
You might think of this in another way as well you might be asked to troubleshoot and something to talk
about too with the with the RSI model is when we receive data we receive data down this physical layer
all the way down to the application when we transmit data.
It goes out the application layer down to the physical when we're troubleshooting this.
It is always best to start with a physical and go down to the application level.
OK.
So say you get a you're working help desk and you get a phone call and somebody says you know my Internet's
not working helped me.
Well what's the first thing you're gonna do or you're gonna ask them application level questions.
Probably not.
You might say hey can you look at the back of the computer.
Do you see a cable plugged in.
Oh the cables plugged in.
OK well do you do you see the where the cables plugged in.
Is there a blinking light is that blinking like green.
By chance OK we're checking the Nick right.
And then we might ask them to you know do they have an IP address what's going on all the way down.
And then we travel issue all the way down to layer 7.
So we wouldn't start to layer seven right.
We would start from the basics and moved down.
So it's important to know this.
This isn't a help desk course by any means but it's super important to notice especially if it's been
a while since you've seen this network stuff or even if this is new to you that the OSA model is commonly
referred to even as a pen tester.
I get all kinds of layer to layer 3 talk and you will be sitting in meetings with network engineers
with people who are very very smart about this stuff and they're gonna throw all this lingo at you.
So if you know this lingo really really benefits you or else you're just going to sit there and wonder
what the heck they're talking about.
So hopefully this is a quick informative method for you.
And again remember please do not throw sausage pizza away.
That's the easiest way I remember it.
You can make up your own harmonics if you want.
People of other things as well if you've got a do you got a favorite pneumonic please feel free to comment
down below and tell me your pneumonic as well so I'd love to hear some of these other ones.
So let's go ahead and move on into subletting and then we'll we'll start moving into other fun parts
of the course.