Join Steve Fullmer for an in-depth discussion in this video The Open Systems Interconnection (OSI) model, part of Windows 8 Networking and Security.
- Before we go into talking about configuring network connections with Windows Eight, we're going to do a quick overview of some network topics. The first one we want to look at is the Open System Interconnector, the OSI Model. So let's do a quick review of that for everybody. Now there are some standard mnemonics, or memory devices, to try to remember what the seven layers are, and I like to start at the bottom, because layer one, right here, is really the bottom one, and we work our way up towards layer seven.
Layer one is the physical layer. With the physical layer, what we're really talking about is actual connectivity. So whether we're fiber connected, we're Ethernet wire connected, we're wireless connected, whatever that physical layer of connectivity is, we sit here at layer one. When we talk about layer one, we're talking about connecting devices using hubs. So we use a hub here at layer one. Our layer two is the data link layer. When we talk about data link layer, we're talking about the way that content, digital content, zeroes and ones, are put together inside of packets to be communicated across the internet.
We have different methodologies for doing that. So for instance, when we talk about data link layer, and we're talking about Wi-Fi, we're talking about eight oh two dot 11, abgn, or we could be talking about Ethernet connections, or we could be talking about the way that we send or encapsulate packets by other standards. So that's the data link layer, layer number two. When we connect devices together at this layer, we talk about switches. A switch is a device that understands the different kinds of communication protocols that can occur between devices.
So typically we can have any device talk to another device using common communication protocols at layer two. Layer three is the network layer. This is the layer where we introduce the concept of IP addressing. So IP addressing on the network here is layer two. We use routers typically to allow communication between devices using their IP addresses. I don't need a router if I'm just going to have all the devices speak to each other in a very small local network.
We can use a switch to do that. Net BIOS, Net BOI protocols have been around for networking since the 80s supported by Microsoft operating systems, operate at layer two. The don't need IP addresses. Microsoft added IP addresses above and beyond that as the internet became available, and actually the first commercial use of internet protocols was in about 92 or 93, that's 1992 or 1993. So that's when we started using IP addressing for boxes to communicate with one another.
Let's go beyond that to layer four. In layer four we're talking about the transport layer. The transport layer talks about how packages are transmitted back and forth. Now I'm going to go to another slide and go into all these layers more deeply, the upper layers, but the separation typically is, as we're talking right here between layers three and four typically, is where we're talking about our connectivity. The infrastructure's layers one, two, and three, when we get to layer four, five, six, and seven we're talking about what the operating system handles for us.
The transport layer's going to be how we bundle actual data or information together. How we cut the pieces of the information up to send packets. We don't send all of the information at once. Because of the model, we are trying to send data in little packets. So let me stop for a second and just talk a little bit about the history and how the OSI model evolved to get a little better context for you. Well the internet evolved pretty much post World War Two as a means for different organizations, particularly military or country organizations, to talk to one another, with the concern that if there was a nuclear holocaust, all kinds of communications would drop.
The goal was to get all data from one place somewhere in the world, or on a battlefield, to somewhere else, maybe headquarters or to another country that was an ally. If we dropped radio communications, we had to get the data there by carrier pigeon, wired, satellite when satellite came along, let's add all of those mechanisms together. So the goal was get the data there. Well if you had one carrier, one piece of information, the whole thing one way and it didn't get there, you'd be in trouble. So the whole goal was to take everything in small pieces and send it out to as many routes as you could to get it to the destination and allow it to be re-assembled once received.
So the transport layer, layer four here, is how we identify how those packets, all the data, is taken into little pieces and distributed, and then when it gets to the other side, how to re-assemble it. Layer five here is referred to as the session layer. The session layer is where, at least from a Microsoft Windows environment, we talk about the difference between ports or protocols and services. So we start a service up typically here, and that service is going to listen for connectivity on a particular port, and that port is going to typically support a protocol, but they're not always bound together.
We'll go into that a little more deeply in just a moment. Layer six is the presentation mode. Presentation is how we're actually putting together the data or information that we are using as users. So we talk about communication format or structure when we're down here in the bottom three layers in the model, we're talking about data or information, usable information to humans rather than just the zeroes and ones of that kind of communication layering.
So up here we talk about the presentation layer, that's layer six. In layer seven we're talking about the application layer. We're talking about the actual applications, the software, that's capable of taking the data and presenting it in an informed or information structure or format. So these are the seven layers of the OSI model. Let's go just a little bit further, and so I've just re-stated the seven here so we can talk about them in a little bit more depth. The ones that I want to focus on here is, we're going to talk about in future models more about the network here, we'll talk about IP addressing in particular, we're going to talk about IP version four and IP version six in some of the next instructional modules that we've got put together, so we're going to talk about the transport layer a little bit more deeply.
In the transport layer we have two different communication modes or methodologies. One is called TCP, user datagram protocol, and the other one's called TCP, transmission control protocol. The difference between the two, and I use this analogy in other sessions, the difference between the two is we're talking UDP, we have large volumes of data that we want to take apart, throw to the destination, and let it be re-assembled. So imagine a jigsaw puzzle. How do you put the jigsaw puzzle back together? You need a datagram, the top of the jigsaw puzzle box gives you a datagram, an idea, of how to put together the puzzle when all the pieces arrive, but the sequence of the arrival of all the pieces is not important as much as that you have all of the pieces, or at least most of the pieces there to re-assemble it.
When we're using TCP, TCP is transmission control protocol, that's where the sequence is important, the word sequence is important, to be able to re-structure the message. So if I said the quick brown fox jumps over the lazy dog, you understand that because of the sequence of words, but if I say dog fox brown lazy jumps quick over, if you hadn't heard the first sequence and you didn't understand what the structure would be, the words just in any order don't make the same sense.
So we use transmission control protocol when the sequence of the data, as transmitted and re-assembled, is important to the structure of the message. We talk at the session layer here, to go a little bit deeper I talked about services when we're talking at the operating system level. When we're talking about data packets we typically talk about protocols or ports. So just going slightly deeper in there, for example, if I was talking about the use of the internet I might say that the protocol was http in a web browser, and port 80 is the common port we use.
But you do not have to automatically link all ports to specific protocols. The first 1023, or 24 ports actually from zero through 1023, are reserved by the standard, but we can use other ports for htt protocol. So it's not uncommon to see port 8000 perhaps, or port 8080 used for the protocol. The important part here, from an operating system perspective, is we're going to use a specific service in order to listen for the protocol to be able to understand it, and we'll configure the service to listen on particular ports.
So here when we say language or format, just continuing our example, we might say html is the particular language format we would use, or html five. Let's go up here to the last layer, the seventh layer of the model, and that's the application layer. The easiest one here when I just continue this is say what application truly am I going to use. So let me stay in this same green color, we're talking about browsers. A browser here, if I'm just continuing this extension, a browser would be the tool used at the application layer to help me see the formatted content.
It's about data. So the layer here is really about data converted into usable information. When I say browsers, in Microsoft Windows Eight space we've got Internet Explorer, so that's the browser that we would bring up to take a look at content. If I go back to my Windows Eight start menu, I can bring up Internet Explorer the metro version. So here's Internet Explorer the metro version. We'll just bring that up for a second.
So this is a browser that's using the seventh layer essentially, the content to be able to send it, and I say the OSI model seven layers to find and function. So you can go out to Microsoft or Wikipedia and find a lot more information yourself to see what is actually containing the details behind these letters. Our goal here was to give you a quick overview of the OSI model. Let me just take this a step further before we transition into networking. Before we go on, let's talk about packets, the actual data that's sent out.
It contains information from all of the seven layers. So if we talk about it, layer one is the physical layer, so that's not inside the data packet, that's the media over which we transmit it. Inside our packets we're going to be putting content here that's from layer two. That is essentially our data link layer, and when we talk about Ethernet protocols we're going to put typically the MAC address, the physical ID, and we're going to have a to and a from identifying the two boxes. So we can have systems that talk to each other without having acquired an IP address.
This would be components from layer two of the model. We put our IP address out here at the very header. So we're going to have a to and a from IP address as well in the header that allows us to communicate between two systems that use IP addresses. All the way down at the end, we're going to have layer seven here, and this is where the data resides. We're going to have to understand what the format or structure of that data is, so inside our packet we're going to have information that tells us something about layer six. So in this case it might say this content is html five.
We're going to have content from layer five that talks about the port that it's running on. So it's going to tell us that this is running perhaps http on port 80, and then we're also going to have to have some information here that helps us understand something about layer four, which is if it's sequenced data we're going to have TCP, and it's going to identify something in terms of the sequence number, how many pieces there are and what is the sequence number. This is an oversimplified view of an IP packet, but to give you a sense of how the IP model works together to send data along with the OSI model using essentially the internet protocol.
So this is just a quick overview of the OSI model before we go into Windows Eight network configuration.