Welcome to the CCNA course! In this section we’ll go over network components; routers switches and firewalls.
Before we really dive into the network components themselves, it helps to go over some network fundamentals so we can keep our conversation more fluid and have some shared definitions while we describe how the components interact with network traffic.
In case you’re not familiar with it, let’s go over the OSI model. Now back in the day when TCP/IP was just in its infancy, we also had another competing protocol stack that was the OSI protocol stack. OSI has its own addressing scheme (NSAP/ISO), just like TCP/IP has IP addresses. A communication functional model was created, the OSI model.
The bottom layer is layer 1, with the top as layer 7. We as network engineers really only care about the first 4 layers. Many engineers will reference anything above layer 4 as “layer 7”. Now when you’re talking with your colleagues and troubleshooting issue it really helps to conceptualize your problems in terms of the OSI layers.
You and your buddy are troubleshooting and you think “it’s a layer 1 issue”, well then we’re looking at a physical cabling issue. This would likely be that you do not have connectivity across a link. Talking about a layer 2 issue you might mean a switching issue or a MAC addressing problem (que the nightmare that is a duplicate MAC address on a broadcast domain) with the frame. You can probably see pretty quick that being able to conceptualize your communication with other engineers in terms of the OSI model layers can be very efficient and useful.
Let’s dive in a little more and take a look at what protocols live at each of these layers, and what their data units are, the word used to define the individual unit of that protocol’s data.
Down at the physical layer we have the physical specifications of wireless (802.11) and wired standards like the usual ethernet connector RJ45. These are shown on the left, with the data unit on the right. The physical layer data unit is typically the ‘Bit’, however in analogue communications, like dial-up, we see ‘symbols’ or a particular frequency which represents multiple bits to increase throughput.
Data Link Layer
Up at the data link layer, layer 2, is where the ethernet protocol lives. This is where we use MAC (Media Access Control) addresses to communicate. The protocol data unit at this layer is a Frame. This is where a network switch does its work. It only looks at each data unit up to this layer before it starts forwarding it along. It will remember the source MAC and the port it came in on and forward it on to whatever the destination MAC address is.
At the network layer is where we start to see Internet Protocol, where we use IP addresses to communicate. Layer 3 is where routers do much of their work. IP routers look at source and destination IP addresses to select the appropriate port to forward the packet out of. We’ll get more into this later, but an important note is that routers will rewrite the source and destination MAC address in the packet header, to be the source of its interface and destination of the next hop, but switches do not modify the frame at all.
Up in layer 4 is where we find out ‘ports’. If you’ve ever heard someone reference that “port 443 is blocked”, or they need to “open port 21 inbound”, they are referencing port numbers that are used in the layer 4 protocols. It’s likely they are referencing TCP or UDP port numbers specifically. While ports aren’t exactly addresses, they are an identifier that the system uses to forward the information appropriately up the application/protocol stack. We’ll get more into this later, and I’ll write up a full lesson on this topic at a later date.
Lets take a look at how an IP packet is constructed, in this case specifically a UDP packet. You don’t need to know yet the difference between TCP and UDP, just that they are layer 4 protocols.
You can probably tell that the layers match up with the OSI model, starting at layer 2 at the bottom. We have the frame header information on the outside of the data unit, then the IP header inside of that, then the UDP header inside of that then our application data.