The Network Layer: Understanding layer 3 of the OSI Model

The third layer of the OSI Model, the network layer, is where most network engineers focus their time and expertise. As Darragh commented in my post on the data link layer, Layer 2 is cool but Layer 3 is the one that can take you places.

Layer 3, the network layer, is most commonly known as the layer where routing takes place. A router's main job is to get packets from one network to another. Layer 3 protocols and technologies allow for network-to-network communications. A Layer 3 switch is simply a Layer 2 device that also does routing (a Layer 3 function). Another key aspect of routers is that each interface on a router has its own IP address, because each of those interfaces is on a different networks. 

So much of what we do as network administrators -- dealing with IP addresses and subnetting, routing protocols, firewall rules and Access Control Lists (ACLs), and many types of Quality of Service (QoS) -- is enabled by Layer 3 technologies. Layer 2 may make you an expert, but Layer 3 is how you get in the money.

When troubleshooting network issues it's helpful to understand if the issue is occurring at Layer 2 or Layer 3 of the OSI model. If you're able to get local communications to work but the packets aren't traversing your Layer 3 boundaries then you've got a Layer 3 issue on your hands.

When troubleshooting Layer 3 issues, Layer 2 technologies can sometimes be a huge help. For instance, if you can't get communications from one side of a router to the distant side of an adjacent router checking for Layer 2 connectivty (like with CDP for instance) between the two devices can give you great insight into the problem. If you've never used CDP (the Cisco Discovery Protocol) it's one worth getting to know as it can be very helpful in many different troubleshooting scenarios.

It's not uncommon when visiting family for me to get asked to fix everyone's computers. The way that I usually respond is that asking me to fix your computer is sort of like asking the guy who designs super highways to fix your car. Yes, cars do run on highways and yes, I do drive a car to work everyday but it's not what I do. This is a great analogy, but there is one key difference between cars on highways and packets on a network and it’s at Layer 3. When you think about how cars get from one place to another –- even with some of the experimental technologies for self-driven cars –- the intelligence that gets the car from one place to another is all in the vehicle. With computer networks, all of the intelligence is at the intersections –- the Layer 3 devices. All the packet (or car in this analogy) really knows is where it’s coming from and where it wants to go. The network does the rest.

While it's important to have a solid foundation in Layer 1 and Layer 2 technologies, you can build a career on Layer 3 expertise. Don't go short here -- go to some classes, read a bunch of books, and most important -- get some hands-on experience with Layer 3 technologies. Like I said, Layer 3 can take you places...

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Josh Stephens is Head Geek and VP of Technology at SolarWinds, an IT management software company based in Austin, Texas. He shares network management best practices on SolarWinds’ GeekSpeak and thwack. Follow Josh on Twitter@sw_headgeek and SolarWinds @solarwinds_inc

The OSI Model Series

Understanding the OSI Model

The Physical Layer

The Data Link Layer (one of my favorites)

The Network Layer

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