There are two basic network models, the OSI Model and the TCP/IP Model.  Both outline the basic functions of how networks work and are each a standard in their own right. Understanding the OSI Reference Model is a requirement for passing any exam on networking whether it is the CCNA, CCNP, Network+ or any Juniper exam.

Memorize the OSI Reference Model. Now. You’ll use it over and over again and none of the information we cover from here on will make any sense if you don’t.  Networks were built one layer at a time over a period of time.  First physical communication was made possible, then logical addresses were used to  allow you to move computers around on a network. Next, the ability to route packets from one network to another was worked out, then ways to guarantee delivery of large volumes of data and finally ways to boot up and to manage the network, provide names humans can remember, and finally to serve up stored data as web pages.

OSI Reference Model’s Seven Layers

There are seven layers in the OSI model.  The entire point behind the OSI Reference Model was to provide a reference standard for networking functions that could be built by any vendor to support what is today a nearly-defunct OSI networking stack of protocols.  Outside of very old networks these protocols aren’t used, but the model still remains a good way to understand networking..

The Internet did away with that stack before it was ever ‘born’, but we still describe the way networks function by using the names of the OSI Model’s layers which live on in the TCP/IP Model’s layers, which are a simplified version of the OSI Reference Model.  This layered model is built around two basic concepts:  first, that each layer operates independently of the other, that is, it’s internal functions have no effect on the layer above or below it.  Second, that the interface, the way in which data is moved from one layer to the next, is standardized. This way, various vendors can write any kind of code they wish, so long as the result is that data is properly handled and passed up and down the stack correctly.

From top down, here is the list of the OSI Reference Model’s seven layers.

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Yes, I really did type this in less than 15 seconds without looking it up and you’ll have to be able to do this just as fast during the CCNA exam.

A Note on Memorizing Anything

First step on learning and memorizing new information is to get to the ‘core’ concepts first.  The OSI model is as ‘core’ as it gets in networking. Memorize this model and you will be able to instinctively understand how two network protocols relate and interact.

Lots of people use a magic phrase to help them remember lists like this such as  “Please Don’t Nuke The Sausage Pizza Again”, but human memory isn’t that simple.  The brain’s stores what our senses perceive, or more precisely, it stores the relationships of what we perceive   This is why it’s easy to remember the Google search you did to find a site, than to remember the name of the site and the page it was on.

Just reading a list several times to memorize it isn’t enough. That only brings in vision.  Memory recall works best if you can provide your brain with more than one relationship to remember something by, starting with your senses, and your brain needs repetition to imprint it properly in the cells.  I memorized the OSI Reference Model’s layers by repeating the names of the layers out loud, adding one layer to the list after repeating everything below it three times:

Physical, Physical, Physical;

Physical, Datalink;  Physical, Datalink;  Physical, Datalink;

Physical, Datalink, Network;  Physical, Datalink, Network;  Physical, Datalink, Network

(and so on)

I did this from bottom up, and then from top down. Then I wrote out the layers the same way on paper while repeating it out loud. Took about 10 minutes to do all this; it was the best 10 minutes I ever spent studying anything IT related.  This technique gets your eyes used to seeing it, your hands used to writing it, your ears used to hearing it, and your mouth used to saying it.  That’s four different ways your brain can recall the list. Once you have memorized the OSI reference model, everything else you learn will be ‘hung’ off one of the layers, and then ‘hung’ off that, building a web of relationships you can remember.  It might be considered ‘brute force’ but it will nail it in your head better than any other technique and you’ll be able to rattle them off forwards and backwards in your sleep.  Besides, you’re going to be using these terms for the rest of your career. There is nothing that reveals a novice faster than getting layers confused, or earns as much disdain from snobby pros.  Knowing this model even helps with troubleshooting, as you’ll soon find out.

The Best OSI Model Tutorial on the ‘Net

InetDaemon.Com’s OSI Model tutorial is required reading at several colleges and universities and websites all over the world link to our OSI Reference Model tutorial.

To get started in internetworking, read the full OSI Reference Model tutorial.

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