From the course: CompTIA Network+ (N10-009) Cert Prep

Ethernet frames

From the course: CompTIA Network+ (N10-009) Cert Prep

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Ethernet frames

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- We've been talking about the concept of frame for a while, and what I want to do in this episode is break down the most important frame type there is, an Ethernet frame. Since Ethernet is so dominant out there, I feel this is the best one for us to break down. Now, I might want to add, there are other types of data that have other types of frames. However, 99% of all the data that moves on the internet moves on Ethernet, so this is the best one to start with. So one of the things that really separates the sheep from the goats, as it were, in the world of networking, are the people who really understand how to break frames down and those who don't. Not only is it important for the exam, but this would be the common banter that network nerds would be tossing back and forth to each other. And more importantly, in other episodes, we actually use tools that grab these frames and allow us to analyze them and you've got to know how a frame is structured to be able to use those tools. Are you sufficiently motivated? Good. Let's go ahead and get started. So what I have here, using my blocks, is an Ethernet frame. Now, of course, this is going to be nothing more than a big old string of ones and zeros, but the blocks represent certain chunks of those ones and zeros in terms of what they do. So we're going to start way over here on the left and march all the way to the right. Starting here on the left is what we call the preamble. Now, if a network card is plugged into a piece of wire, how does it know a frame is coming? And that's the job of the preamble. So the preamble is nothing more than a bunch of alternating ones and zeros. So it's like me, ma, me, ma, me, ma, me, ma, here comes a frame. So the network card wakes up, pulls out his baseball catcher's mitt and starts grabbing the rest of the frame as it comes in. So that's the preamble. Now the next, and this is the destination MAC address. So we have to have some kind of addressing to know where it's going, and that's what that carries right there. So that's a 48-bit MAC address that's built into every network card in the universe. After that is going to be the source MAC address. Now, if you're going to be sending somebody data, there's probably a good chance that they might want to return something back to you. So by giving them the return address, whoever you're sending stuff to, knows where it came from, and then they can send something back if they need to. Now, right here is what we call data type. In fact, if you want to be more technically correct, we would call this the ether type. What this little guy's doing, and he's only about two bytes long, and his job is to let us know what kind of data we're hauling. So he'll have values like 0800, for example, means that I'm sending IPV4 data, or there's IPV6 or ARP. All of these different things are discussed in other episodes, so don't worry if you don't know what those are yet. Next is the data itself. Now the data is kind of an interesting thing because you have a minimum amount of data that you can haul at a maximum amount of data. So, within the world of Ethernet, you can haul about 64 bytes. And by the way, we don't even use the word bytes when we're talking about this stuff. We use the word octets. A byte and an octet is still eight bits, okay? But sometimes you'll see the word octet instead of byte. Don't worry about it. Same thing. So the minimum amount of data it can haul is 64 bytes. If, by some chance, you're hauling just a little tiny piece of data, like you're saying, "Okay, got it," which doesn't mean that much to anybody else, we're going to add something called a PAD. Now, the PAD pushes this up to the absolute minimum size. So if you've got a tiny little bit of data, you add a PAD to get it to 64. Now, on the maximum side, and this is also very important, the maximum amount that you can haul is about 1,522 bytes or octets. That's interesting to me because, come on, we watch YouTube videos, right? We're doing online gaming. I'm buying a whole bunch of new barbecue stuff at eBay, whatever it might be. So we've got a lot more than 1,522 bytes of data sometimes. That's absolutely true. And there's no problem, all we do is we just send a bunch, we just chop up whatever chunk o' data you got into nice 1,522 byte pieces. And we've got something in here called a sequencing number, so, like, ever get like 30 boxes and it'll say box one of 32 of 33 of 30 kind of a thing? We have the same thing built into networking and it takes care of all that for us automatically. So 1,522 bytes or octets is the most data that you can haul within one individual frame. However, with some of the more advanced types of Ethernets that we're seeing today, they have something called a jumbo frame. A jumbo frame is unique and it's only for really high speed stuff but it can push one individual frame up to be able to haul as much as 9,000 bytes. So jumbo frames do come into play but only for very high speed networks. So this value, the maximum amount of data we can haul, is known as an MTU or maximum transmission unit. All right, as you might imagine, it might be a job for a frame to go from one host and through some wires and through some boxes and through some more wires, and who knows what else before it gets to the other host. In that time period, things can happen. We could have lightning strike or wires get cut or people start turning on blow dryers or anything that could actually degrade that signal. So the last thing we do with any type of Ethernet frame is this guy right here on the end. This is called the frame check sequence. The frame check sequence is what we call a 32 bit cyclic redundancy check, which is basically groovy binary mathematics that says to us, "Check this value out, do some math, and if you don't get this value, then something's wrong." And then whoever gets it's going to go, "Sorry, it's messed up. Please resend." So these are the main chunks of every Ethernet frame that are out there. It's important that you take the time to memorize each one of these and have a basic understanding of what they do, not only for the exam, but for real world network troubleshooting.

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