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  • As a system administrator what should I know about the OSI model? (Rec X.200)
  • Briefly, what are the layers and what do they do?
  • How much do I need to know about in my day-to-day work?
  • How does this contrast with the TCP/IP model? (RFC1122)
  • Does the OSI model still apply on today's networks?
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9 Answers 9

up vote 15 down vote accepted

I don't see anything wrong with being able to differentiate layers between the two models. On the plus side, having knowledge of the OSI model comes in handy when you're talking to people or reading documentation referencing the top 4 layers of the OSI model:

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Here are some links with explanation on what each layer does:

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Your numbers in the right hand column are not helpful - they may lead people to confuse which "layer" people are talking about when they say "layer 2" (which IMHO always means the Data-Link layer). –  Alnitak May 20 '09 at 14:50
    
What right-column numbers? What are you talking about? ;) Thanks for pointing that out! –  l0c0b0x May 20 '09 at 15:46

As techs we use the OSI model all the time as a network tech when trouble shooting a network connection. Since our networks where build on this model we use it even if we aren't really aware of it.

When troubleshooting a network problem we often go up the OSI mode.

  1. Physical - Is the network cable plugged in?
  2. Data Link - Do you have a link light?
  3. Network - Are you getting an IP?
  4. Transport - Can you ping your default gateway?
  5. Session - Do you have DNS server information? Can you ping 4.2.2.2 but not google.com?
  6. & 7. Presentation & Application - Can you browse to a site?

You use the OSI even if your not aware of it being aware of it. Being aware of it might help you communicate better with people/vendors about network situations. Many companies require extensive OSI knowledge for a networking certification (I know Cisco did).

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1  
ping is ICMP and so L3 (if it's not arping or tcping, those L2 and L4 respectively). Better example is telnet, i think. –  SaveTheRbtz Nov 22 '09 at 11:12

You need to know the OSI model because a lot of documentation for proxies, load balancers and other such things will refer to "Layer 7" or "Layer 4", and you need to know what they are talking about.

That wiki page does a pretty good job explaining the layers. The TCP/IP model sits on top of the OSI model.

As to whether it applies today, the answer I'd say is, "sort of". A lot of devices these days are hybrids and work on multiple layers at the same time to route packets more efficiently. However, if you are doing any sort of network design, knowing at least the basics and some of the key acronyms for at least layer 2,3,4 and 7 are handy.

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I got my first job in 1990 by reciting the OSI layer model. They were most impressed. I didn't actually understand it but boy could I recite it well.

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These days, you don't need to know the whole OSI model.

You really just need to know:

  • Layer 2 - ethernet, wifi, ppp, etc frames
  • Layer 3 - IP packets
  • Layer 4 - TCP and UDP packets
  • Layer 7 - everything else

(invert to taste, if it offends you to have Layer 7 at the bottom)

See also https://www.isc.org/logoware/isc-9-layer-osi-model-cotton-t-shirt

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I find my knowledge of the OSI model incredible useful in solving networking and system issues. It is a theoretical model one can use to guide investigation into a problem even though TCP/IP does not directly map to the OSI model.

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You can find information about OSI model by visiting http://www.linuxconfig.net/basic-information/osi-model

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As a system administrator myself, proficiency of the OSI model helped me a lot in identifying problems on my network. The link below is a simple explanation on how you could use the OSI model to troubleshoot networks.

http://www.petri.co.il/csc_how_to_use_the_osi_model_to_troubleshoot_networks.htm

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I genuinely don't believe that many, if any, sysadmins need to know the OSI model in any specific way, however the IP model is staggeringly relevant and is used all the time.

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