TechKnowSurge
CompTIA Network+ 1.1 Cisco CCNA 1.1 Cisco CCST Networking 1.1 Cisco CyberOps Associate 4.8
TechKnowSurge Catalog Videos The OSI Model
VideoNetworkFree

The OSI Model

The OSI model is a seven-layer framework that standardizes how network protocols interact, enabling interoperability, structured troubleshooting, and technology upgrades across different protocol suites. It provides a common reference for understanding how data is packaged, addressed, and transmitted between networked devices.

⚑ Complete this video to capture a CTF flag worth 1 point.

About this video

The OSI model is a seven-layer reference framework designed to standardize how network protocols are organized, how they interact with one another, and how data moves across a network from one device to another. Developed in 1984, it emerged from the need to bring structure to a landscape of diverse protocol suites built by different organizations and vendors, often with no shared conventions. By grouping protocol functions into defined layers — physical, data link, network, transport, session, presentation, and application — the model gives engineers and developers a shared vocabulary and a clear picture of which layer is responsible for which job. The practical value of the OSI model shows up in three key areas. First, it makes education more tractable by allowing each layer's responsibilities to be taught and understood in isolation. Second, it promotes interoperability by establishing how one layer should interface with the layers above and below it, making it possible for protocol suites developed independently to work together. Third, it supports modular upgrades: because the interfaces between layers are defined, a single layer can be replaced or improved without requiring changes to the rest of the stack. In practice, data moves down through the layers on the originating device — a browser request, for example, passes from the application layer down through presentation, session, transport, network, data link, and physical, with each layer adding its own header or framing information. The resulting frame is transmitted across the network and then unpacked in reverse order on the receiving end, with each layer processing and stripping its relevant header before passing the data upward to the next layer. The OSI model is a conceptual tool rather than a strict implementation standard, and many real-world protocols — Ethernet being a well-known example — do not map cleanly to its layer boundaries. Understanding both its usefulness and its limitations is essential to applying it effectively in network analysis and design.

What you'll learn

What's covered

OSI Model

Aligned to

CompTIA Network+
1.1 Explain concepts related to the Open Systems Interconnection (OSI) reference model.
Cisco CCNA
1.1 Explain the role and function of network components
Cisco CCST Networking
1.1 Identify the fundamental conceptual building blocks of networks
Cisco CyberOps Associate
4.8 Interpret the fields in protocol headers as related to intrusion analysis

Key terms

Open Systems Interconnection Model
OSI
A conceptual framework that standardizes network communication into seven distinct layers.
Protocol
A set of rules governing the format and transmission of data between devices.
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers of the OSI model.
Physical Layer
Layer 1 of the OSI model, responsible for the transmission of raw bits over a physical medium.
Data Link Layer
Layer 2 of the OSI model responsible for node-to-node data transfer and error detection on a single network segment.
Network Layer
Layer 3 of the OSI model, responsible for logical addressing and routing data packets between networks.
Transport Layer
Layer 4 of the OSI model, responsible for end-to-end communication, flow control, and error recovery.
Session Layer
Layer 5 of the OSI model, responsible for establishing, managing, and terminating communication sessions.
Presentation Layer
Layer 6 of the OSI model, responsible for data translation, encryption, and compression.
Application Layer
The topmost layer of the OSI model (Layer 7) that provides network services directly to end-user applications.
Protocol Data Unit
PDU
The unit of information passed between peer entities at the same OSI layer, with layer-specific names: segment or datagram at the Transport layer, packet at the Network layer, frame at the Data Link layer, and bit at the Physical layer. Understanding PDU terminology is fundamental to troubleshooting network communication at each layer.
De-encapsulation
The process of removing protocol headers from data as it travels up the OSI model layers on the receiving device.

Transcript

The OSI model is a structure that helps us understand different protocols and how they interact with each other. It helps us understand these protocol suites, and it helps us do things like standardize how these protocols interact with each other.

Protocols are like puzzle pieces, and they're really designed to interact with certain puzzle pieces. But the more other puzzle pieces they can interact with, the more interoperability we have, the more innovation we can have, the more we can connect to different technologies. So it really is to our advantage to create some sort of standards that we can work off of. That is one of the reasons why we created the OSI model.

Building up the layers

I'm going to use these Duplo Legos to help illustrate the OSI model and its advantages. First of all, we have our data that needs to be delivered, so this one represents the data to be delivered. Then we have our different protocol suites, so each one of these colors is a different protocol suite. Then we have the different functions our protocols are carrying out, and some of these are grouped up. For instance, let's say we have a protocol that does all three of these: segmentation, connection, and error detection. What the OSI model does is allow us to group these into different layers, so that we can better see what different layers do and the functionality of each of those layers.

This has several advantages. One is that it allows us to teach what the DEH layer does and what its purpose is. Another is that it promotes interoperability. Each one of these protocol suites could be developed by a whole different set of people in a whole different country, and have a very different way of interacting. It's not like Legos that are meant to click together; they have very different ways in which they match up and work together. But by laying this out in this type of structure, we can better create these linkages between the different layers. So now this protocol suite that does addressing really well can match up with this protocol suite that does segmentation, connection, and error detection really well, which can be matched up against confidentiality of this protocol suite. Now all of them can be created to interconnect together.

Finally, the OSI model promotes the ability to do upgrades really well, because now there's a standard in how these two layers interconnect with each other. By establishing how that happens, maybe we need to upgrade our segmentation, connection, and error detection to be the latest and greatest. Now we can take that out and implement it with the newest level of technology without disturbing the other layers. This all happens because the OSI model helps define the different layers and the responsibilities of each of these layers.

The model and its limits

Back to our network communication protocol map, the one where we have our different protocol suites across the top and, across the left, the OSI model and the different layers: layer 7, 6, 5, 4, 3, 2, and 1. There are some common functions of each one of these layers, what job it's to perform, and how they're interconnected between these different protocol suites. This is a way that we can visualize the OSI model and the interconnection between these different protocol suites.

One thing to understand is that, as much as we'd like to think it's as clean-cut as this, it is just a model. The model came out in 1984. A lot of these protocols were already well developed by 1984, so even though we've broken it down this way, things don't always fit great. Ethernet is a great example of that. Here we have our layer 3 across the top, the network; layer 2 is this data link; and layer 1 is the physical. As we can see, Ethernet doesn't really follow the same boundaries as the OSI model. So while the OSI model is a great model to work with, it's not perfect.

Encapsulation across the layers

The OSI model is really designed to work from the top layer and work down through the layers on one side only, to be unpacked on the other side. What do I mean by that? Here we have a computer, and let's say on the computer we have a browser open. We open up Chrome, and that is the application layer. Chrome does the presentation of the website to us, and we make some sort of request through the Chrome browser. The request is going to be for some sort of HTTPS service; it's going to be a web service. Over here on the server side, we have a web service that's running.

So the application, Chrome, needs to make the request. It goes to the presentation and session layer. At the transport layer, that request might be really large, so we break it into smaller pieces; that's one of the jobs of the transport layer. It needs some addressing, so it adds some addressing in the network layer. It also needs to communicate across the local network, so we add the data link and physical layer. It builds up this frame that's going to be sent across the local network. It gets sent over, and many things can happen during this process. Then essentially it gets unpacked. When this device gets it, it processes the data link and physical layer to make sure that it's destined for this machine. Once it does that, it no longer needs that and unpacks it. It gets to the addressing and the network side and processes that. Once it is done with that, it gets rid of it, takes the transport layer, and processes that, and works its way back up to the application, where it hands off to the web service, maybe in this case Apache, to form a response. It builds a response to that request, builds it back up to be sent back over, where this machine then needs to unpack it on its side.

The layers and the protocol data unit

This table illustrates the differences between each one of those layers. We have layers 7 through 1, and then the name of each. It starts at the application layer and works its way to the physical layer. In this diagram we have examples, which we'll go more and more in depth into, of each one of these different protocols and what the functions are. We also have the protocol data unit, what it's called and what it looks like visually. We just start out with data for the first few rounds. We add the TCP header to the data. Then we have addressing that needs to be added from a global perspective, and then the local addressing that gets added. So we add this up and then put the bits across the lines with the ones and zeros.

About TechKnowSurge

TechKnowSurge builds IT and cybersecurity professionals through hands-on, concept-first training built around real understanding — not memorization. Free interactive tools, structured programs, and 25+ years of real-world experience, all in one place.

Explore free tools and programs →