Certified: The GIAC GISF Audio Course

In this episode, we shift from cryptography into networking fundamentals, because a lot of cybersecurity depends on understanding how data actually moves from one device to another. Many new learners feel intimidated by networking models because they are presented as memorization tasks, but you do not need to treat them that way. The OSI model and the TCP IP model are simply ways to organize the complicated job of moving data across networks into layers with clear responsibilities. When you have a mental model of data flow, you can reason about where security controls operate, where attacks happen, and what kind of evidence might exist in a network event. For the G I S F exam, you need to be able to connect basic network behavior to security concepts without getting lost in jargon. Our goal is to make the layered models feel like a helpful map rather than a list you cram and forget.

Before we continue, a quick note: this audio course is a companion to our course companion books. The first book is about the exam and provides detailed information on how to pass it best. The second book is a Kindle-only eBook that contains 1,000 flashcards that can be used on your mobile device or Kindle. Check them both out at Cyber Author dot me, in the Bare Metal Study Guides Series.

Start with the reason layered models exist. When you send data from an application on your computer to an application on another computer, many tasks must happen. The data must be formatted so the receiving application can understand it. It must be broken into manageable pieces, addressed correctly, routed across networks, and delivered reliably. It must also be converted into signals that travel across physical media like cables or wireless. If all of those tasks were handled in one big tangled system, it would be hard to design, troubleshoot, or secure. A layered model separates responsibilities so each layer can focus on a specific set of tasks while relying on other layers for support. This separation makes networks scalable and predictable, and it also helps security teams decide where to apply protections.

The OSI model, which stands for Open Systems Interconnection (O S I), is commonly described as seven layers. You do not need to treat the number seven as the most important part; the important part is what the layers represent. The lower layers deal with physical transmission and local delivery, while the middle layers handle routing and reliable transport, and the upper layers handle application-specific communication. When data is sent, it moves down the layers on the sender side, is transmitted across a network, and then moves up the layers on the receiver side. Each layer adds its own information, often called headers, that help the data move correctly and be interpreted correctly. This process is sometimes called encapsulation, where each layer wraps the data with information needed by that layer’s function.

At the bottom of the OSI model, the Physical layer is responsible for sending raw bits as signals over a medium. This could be electrical signals in a copper cable, light pulses in fiber, or radio waves in wireless. The Data Link layer sits above Physical and is responsible for local network delivery on a single segment, such as between devices on the same local network. This is where concepts like frames and hardware addresses belong. The key idea is that Data Link focuses on getting data from one device to another on the same local network, while Physical focuses on how those bits actually travel. Many security issues at these layers relate to access to the network itself, such as someone plugging into a network port or interfering with wireless signals.

The Network layer is where routing enters the story. Routing is the process of moving data from one network to another, often across many intermediate devices. This layer deals with logical addressing so data can be directed toward the correct destination network even if sender and receiver are far apart. When you hear about Internet Protocol, you are in this area of responsibility. The Network layer’s job is to choose paths and forward packets toward their destinations. From a security perspective, this is where you think about controlling which networks can talk to which other networks, and where you might see issues like spoofed addresses or traffic being routed in unexpected ways. Understanding that routing happens here helps you place certain threats and controls in the correct part of your mental map.

Above Network is the Transport layer, which is responsible for end-to-end communication between applications. End-to-end means from the sending device all the way to the receiving device, not just across one network hop. Transport handles concepts like ports, which allow multiple applications to share a network connection without mixing their data. Transport can also provide reliability, meaning it can ensure data arrives intact and in the correct order, depending on the protocol used. This is where Transmission Control Protocol (T C P) belongs, along with User Datagram Protocol (U D P). The key distinction is that some transport methods focus on reliable delivery, while others focus on speed and low overhead. Security questions often connect to Transport because many controls and attacks reference ports and services, and because certain attacks exploit how transport sessions are established.

Above Transport, the OSI model includes Session, Presentation, and Application layers, which are often grouped together conceptually for beginners. Session involves managing the ongoing communication between systems, such as establishing and maintaining a conversation. Presentation involves formatting, encoding, and sometimes encryption, ensuring data is represented in a usable form. Application is where application-specific protocols and services operate, such as web browsing and email. In practice, modern networking stacks do not always separate these layers as neatly as the OSI model describes, but the model still helps you reason about where functions live. From a security standpoint, many threats and controls operate at the upper layers, such as application vulnerabilities, authentication, and data validation. Understanding that the upper layers are closer to user activity helps you connect them to common attack scenarios.

Now let’s introduce the TCP IP model, which is a more practical model used to describe how the internet works. It is often described as four layers: Link, Internet, Transport, and Application. You can see how it maps to OSI by grouping OSI layers. The Link layer roughly corresponds to OSI Physical and Data Link. The Internet layer corresponds to the OSI Network layer. The Transport layer matches OSI Transport. The Application layer covers OSI Session, Presentation, and Application. The key value of learning both models is not memorizing two separate lists; it is being able to translate between them and understand that they describe the same overall process with different granularity. On the exam, you may see either model referenced, so comfort with the mapping is helpful.

To build a mental model of data flow, picture a message created by an application, like a request to view a webpage. The application creates the request data. That data is handed down to the transport process, which adds information so the receiving system knows which application should receive it, often using port numbers. Next, the internet or network process adds addressing so the data can be routed to the correct destination network. Then the link process prepares the data for local delivery on the next network segment, using local addressing and framing. Finally, physical transmission sends the bits across the medium. On the receiving side, each layer reads and removes the information intended for it, then passes the remaining data up until the receiving application gets the original message. This down-and-up flow is the heart of layered networking.

Encapsulation also helps you understand what network evidence looks like. When security tools capture traffic, they often see the outer layers first, like link and network headers, along with transport information such as ports. This information can reveal which systems communicated, what protocols were used, and how data traveled. If encryption is used at higher layers, the payload may be unreadable, but the metadata can still provide valuable clues, like timing, destination, and session behavior. This is why understanding layers matters for security analysis. You can recognize what you can and cannot see depending on where encryption and controls operate. Exam questions sometimes test this by asking what kind of information is exposed even when traffic is encrypted.

Layer models also help you place security controls correctly. Some controls operate at the network level by filtering traffic between networks. Some operate at the transport level by controlling which ports are open and which services are exposed. Others operate at the application level by validating input, authenticating users, and enforcing business logic. When you hear terms like network segmentation, you should think about controlling communication at the network layer. When you hear about blocking a service, you should think about ports and transport behavior. When you hear about preventing injection attacks, you should think about application layer processing. You do not need to memorize every control, but you should be able to connect a control to the layer where it acts. That connection is often what exam questions are testing.

A common misconception is that the OSI model is purely academic and not relevant. Even if real-world networking stacks blur layer boundaries, the model remains valuable for communication and troubleshooting. It gives you a shared language for describing problems and protections. Another misconception is that protocols perfectly align with one layer. In reality, some protocols span multiple responsibilities, and security features can be implemented at different layers depending on design. The exam is not usually asking you to debate edge cases; it is asking you to use the model as a practical reasoning tool. If you keep your focus on each layer’s job, you will be able to answer most foundational questions confidently.

As you study data flow, keep linking it back to security objectives. Confidentiality is often protected by encryption, which typically operates at upper layers or in specialized secure protocols. Integrity can be supported by checks at various layers, from link-level error detection to application-level validation and cryptographic signatures. Availability depends on robust network design and protection against disruptions, which can involve multiple layers. Recognizing that security is layered just like networking reinforces the idea of defense in depth. When an exam question describes an attack, ask which layer is being targeted and which layer a control would best defend. That simple habit turns layered models into a practical decision tool.

To conclude, the OSI model and the TCP IP model provide structured ways to understand how data moves across networks. The OSI model breaks communication into seven layers, while the TCP IP model groups functions into four layers used to describe internet communication. Data flows down the layers on the sender side through encapsulation, travels across networks, and flows up the layers on the receiver side through decapsulation. Each layer has specific responsibilities, and understanding them helps you place threats, evidence, and controls in the right context. If you carry one decision rule from this episode, let it be this: when you face a networking security question, first identify where in the data flow the problem occurs, then choose the layer and control that directly address that part of the communication path.