What is the OSI model and how does it apply to todays networks?

• 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?

Solution 1:

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:

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

• http://www.dummies.com/how-to/content/getting-to-know-the-osi-model-for-the-ccna-exam.html
• http://www.webopedia.com/quick_ref/OSI_Layers.asp

Solution 2:

As techs we use the OSI model all the time as a network tech when trouble shooting a network connection. Since our networks were built 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 model.

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 (or more more accurately as SaveTheRbtz pointed out telnet to) 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 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).

Solution 3:

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.

Solution 4:

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.

Solution 5:

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