OSI Model vs. Exmples on CBT Net+ nuggets

Hi everyone,

I am studying for the Network+ and utilizing Keith Barker's CBT nugget videos.

After diving in several videos already, I am still puzzled about the OSI model.

The OSI model shows that data goes from Host PC down layers Application> Layers 4-3-2-1, etc and back up to the destined PC from layers 1-2-3-4, > Application, etc.

In one of the examples, Keith Barker shows a PC connected to a switch, and switch to the router before it goes out of the network to a DNS server.

How can the data from the PC be sent out from PC to layer 2 (switch) to layer 3 (router) and out of the network? Because isn't the OSI model sequence out of order if that PC sends data from Application to Layer 4, then Layer 2 (the switch in the example) then to Layer 3 (router) and out of the network? I am confused on how nugget shows the PC sending data to a layer 2 device before reaching the layer 3 (router), seems to be switched around..

:

If someone could please explain, I'd very much appreciate it.

Thanks so much for your help!

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Comments

jaywalker

I don't want to confuse you further so I wont try to explain

I'm studying the Network+ myself and have seen some of his CBT Nugget videos. Have you checked out Professor Messer's Network+ videos? His explanation of the OSI model is easy to understand and it may clear things up for you. Good luck!

Fulcrum45

The OSI model is followed sequentially by each device that receives the data. Nothing is really out of order in that example it just looks that way. Your PC uses all 7 layers of the OSI model before sending. From there it passes it on to a switch that only needs to read up to layer 2. From there it then passes it on to a router that needs to read only up to layer 3 (and to some degree layer 4- but we won't go into that). From there it eventually passes it on to the DNS Server which will in turn run up the layers to 7. All the devices read the OSI model in order but the example you saw made it seem that it was out of order. Maybe a quick diagram helps:

Your PC /Switch/ Router
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3 /3
Layer 2 /2 /2
Layer 1 /1 /1

I hope I explained this ok

Fulcrum45

I tried to make the table a bit easier to read but it wont let me save it the way I wanted.

luongx327

Thanks alot Fulcrum45, that helped alot!

My only question is you posted:

Your PC /Switch/ Router
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3 /3
Layer 2 /2 /2
Layer 1 /1 /1

But should it be like this?

Your PC /Switch/ Router
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3 / /3
Layer 2 /2 /2
Layer 1 /1 /1

Because the switch is Layer 2, I dont understand how you included the 3 in your original post.

Fulcrum45

Whoops! I thought I fixed that- you are correct, the switch should only be going to layer 2 and the router goes up to layer 3.