Please Whitelist This Site?
I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)
If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.
If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.
Thanks for your understanding!
Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide
NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.
How To Use The OSI Reference Model
I have been trying to make the point
in this section that the OSI Reference Model is a very important tool
in understanding how networks function. However, while some people tend
to play down the OSI model too much, suggesting that it really isn't
very relevant today, there are others who go to the other extreme. They
use it too much, overanalyzing and trying to use the model in a way
that was never intended.
The most common mistake is attempting
to try to make everything fit into the layered structure
of the OSI model. (I must confess to falling into this trap myself on
occasion.) When I first started laying out the structure
of this Guide, I wanted to organize everything
based on where it fell in terms of OSI model layers. I quickly discovered
that this was like attempting to put pegs of various shapes into a board
containing only round holes. I had to change my approach. I ended up
organizing the Guide based on the OSI layers where it made sense, and
using a different structure where it did not.
I recommend that you learn from my
experience. A simple rule of thumb is this: refer the OSI Reference
Model if it helps you make sense of technologies and understand how
they work; don't use it if it makes things more complicated
instead of more clear. In particular, bear the following in mind:
- It can be very hard to even figure out where
some technologies fall within the model. Many protocols were designed
without the OSI model in mind, and they may not fall neatly into one
layer or another. Some overlap two or more layers; other protocol suites
may have two protocols that share a layer.
- The boundaries between the upper layers (session,
presentation and application) get particularly fuzzy. Some protocols
are clearly designed to fit at one of these layers, while others may
overlap all three. It is for this reason that I
do not categorize higher-level protocols by layer in this Guide.
(The OSI Reference Model was in fact designed to take into account the
fact that differentiating between these layers might not make sense.)
- The OSI Reference Model was designed primarily
with LANs in mind. WAN technologies often fit very poorly into the OSI
model, with lots of overlapping and partial layer coverage. However,
it's still useful in most cases to look at these protocols in terms
of their approximate fit in the OSI model, since parts of WAN technologies
are sometimes interchanged.
- The people who design products don't generally
worry about ensuring that their latest inventions implement only specific
layers of the model. Thus, sometimes new products come out that break
the rules and implement functions across more than one layer that
used to be done by multiple devices at the individual layers. This is
usually progress, a good thing. J
Finally, an observation
noticed that people learning about networkingespecially those
trying to memorize easy answers to difficult questions so they can pass
examsoften ask at what layer does this piece of hardware
operate? The problem here is not the answer but rather the question,
which is simplistic. With the exception of simple physical devices such
as connectors and cables, pretty much all networking devices
operate at many layers. While a router, for example, is usually associated
with layer three; it has two or more device interfaces that implement
layers two and one. A better question is what is the highest
layer at which a device functions.
The bottom line is that the OSI Reference
Model is a tool. If you use it wisely, it can be immensely helpful to
you. Just remember not to be too inflexible in how you apply it, and
you'll be fine.
Key Concept: It is just as much a mistake to assign too much importance to the OSI Reference Model as too little. While the model defines a framework for understanding networks, not all networking components, protocols and technologies will necessarily fall into the models strict layering architecture. There are cases where trying to use the model to describe certain concepts can lead to less clarity rather than more. One should remember that the OSI model is a tool, and should be used accordingly.
|If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!|
Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.