The OSI Model - Just break networking down

The most fundamental, the most basic aspect of networking has to be the OSI model. The OSI model is one of the first things taught in a networking class and is referred back to it constantly in the higher tier classes and even in the work environment. The model has 7 layers that take control of each step a piece of data goes through before it's going from point A to B. This is a great picture that gives a good overview of the layers. Almost all protocols sit on one layer of the OSI model, this may not seem like a big deal, but when troubleshooting network related issues it is crucial for engineers to narrow down the issue to a particular layer of the OSI model. 

For example,  I had one outage that a route for a customer was down. We traced the issue down to 2 routers (operate at layer 3). Now we  assumed it was a problem with the routers and were trying to investigate off that, however we came to find out the issue were the switches (layer 2) in between of which there were 2. When you are looking at a particular layer you're only looking at that layer and if there is an issue in the layer below you won't know. From everything we saw the layer 3 was perfect, and it was layer 2 which is where mac addresses operate that had issues. 

Now generally, networking classes spend majority of the time on the bottom 4 layers, and especially layers 2-3. The only thing you really need to know about layer 1 is that it is the physical layer and it controls the physical transmissions of electricity, light or waves depending on how data is being transferred, be it Ethernet cable, fiber or wireless. Top 3 layers I'll be honest I don't work much with, however recent network trend is shifting towards utilizing these upper layers to control the bottom ones so hopefully I'll have something for you all in the near future. Layer 2 is where MAC addresses lie, MAC addresses are unique for each interface of each device in the world. There are no 2 mac addresses that are the same (except virtual MAC addresses). Switches operate at layer 2 level and host a table matching interfaces to MAC addresses. 

Layer 3, the most popular layer amongst network troubleshooting is where the IP addresses operate at. Layer 3 are also where basically all routing protocols operate at. Layer 3 is where you partition out your network into subnets. There is a lot more to be said about both layer 2 and 3 especially along with the other layers, but those would each be in there on separate post. If there is something that was not clear or you guys have any other questions regarding networking or tech let me know below.