OSI Model for beginners…

by Chris Roberts

Thought it was time to update this based on the number of folks coming into the IT/InfoSec/Cyber arena….enjoy:

Layer1: Physical

The hardware, the juice, the bits that zap you if you test them with your tongue…this is where TRON happens…This is the LEGO of the world we live in today. Welcome to the fundamental building blocks of stuff that makes everything else happen. Consider it the foundations of the house, and as with foundations it’s got some cracks…

Layer2: Data-link

Think of this as the post office, it takes your barely legible hand written address, adjusts it to make sense and then hands it off, hopefully in the knowledge that it makes sense to everyone else. Now, it can only hand it over to something close by, so it’s like the regional post office…and thankfully it’s got the Layer3/FedEx to handle the long distance stuff…

Layer3: Network

This is the FedEx layer of the network, you know your package has to get to Tasmania, you have no freaking clue how it gets, there, but this layer works it out, plans it accordingly and boots your package on… This is basically the same as getting on a plane, you know where you are going to end up, but you don’t fly the damm thing, and you are NOT talking to the traffic control…that’s handled by the folks at the pointy end…

Layer4: Transport

This is the Intermodal of the networking world….somehow fluffy bears made in a sweat shop 1/2 way up a mountain in the far east actually DO arrive on the shelves of the store in time for whatever hallmark holiday’s next in line. This layer works out which bears go where, how many to send and what frequency to send them. If this screws up, you get Valentine bears in December. 

Layer5: Session

Sometimes you have to just reach out and touch someone (Amazon, eBay, Instagram, Tube8) this is the layer that makes sure you CAN talk with anyone, everyone, someone out there and that the world might actually be listening, or at least ready to take your money. 

Layer6: Presentation

Think of this as the overworked, stressed out, caffeinated stage hand at a Broadway musical…ALL the sensory inputs are being listened to, the musicians complaining, the dancers pouting and NOBODY understands what the hell the Soprano’s on about…this is the layer that makes sense of it all, sorts it out and get’s it ready.

Layer7: Application

The squishy bit at the top, the icing on the cake, the calm duck on the millpond of life…this is the part everyone “sees” the pokey finger on the phone through to driving the mouse all over the Googles… This is what makes it all look pretty and this (unfortunately) is where so much of the glitz is….sometimes at the expense of everything else.


This is the squishy layer, the bag of calcium and water held together by 100,000 miles of blood vessels. This is you, I, and senile old Uncle Bob who still thinks that shouting at Siri will get her to adjust the thermostat. This is arguably one of the most misunderstood layers out there as the bloody parameters change on a person to person, day to day basis…and the geeks have yet to work that out.


This can often be represented as the company, the policies, procedures and controls that an organizations vainly attempt to overlay onto of the other 8 layers, often in a hope of trying to manage, control OR simply understand WTF is going on… Often with a vendor or supplier trying to sell some blinky shit that “removes the need to worry about any of it”


This is the stoning ground of NSA, NRO, NGA, GCHQ, Israel, 3PLA, Etc… consider the fact they have their hooks into Layers 1-9, this is just the cherry on the top….this is the governmental layer.

‘all for now


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