In this lesson, we will cover the fundamentals of computer networks. Understanding how networks operate is an essential skill for every system administrator. Today, the majority of computer systems communicate with each other over networks, rather than using physical media such as USB drives. Networks span the entire globe and provide data exchange, resource access, remote management, and support the operation of modern services. A solid knowledge of networking technologies allows you to efficiently configure and secure infrastructure, as well as quickly identify and resolve potential issues.
In this section, we will take a detailed look at the physical layer of network models. The physical layer is responsible for the transmission of bits over physical media — cables, fiber optics, and wireless channels. At this layer, you will learn about cable types (twisted pair, fiber optics, coaxial), connector types (RJ-45, LC, SC), transmission standards (Ethernet, Wi-Fi, Fast Ethernet, Gigabit Ethernet), as well as network adapters and hardware used for data transmission.
Understanding the physical layer is essential for selecting and installing equipment, troubleshooting network issues, and ensuring stable and high-speed data transfer. We will cover both theoretical concepts and modern standards (such as 10G Ethernet, PoE, Wi-Fi 6), cable management best practices, and proper organization of server rooms and workspaces.
Special attention will be given to how network configuration and management are performed in Linux-based operating systems. In addition to practical Linux aspects, this course covers the key topics included in the internationally recognized Cisco CCNA certification, providing a solid theoretical foundation and hands-on skills for professional development in networking. The knowledge gained will be valuable not only for Linux, but also when working with other operating systems and network hardware.
The format of this course imposes some limitations on the inclusion of hands-on tasks in the initial lessons. As a result, a substantial portion of the theoretical material will be presented without immediate practical exercises. It is recommended to study the theory carefully, as it provides the foundation for mastering practical skills later on. Be prepared for a focus on core principles and terminology at this stage.
In the second part of the networking course series, the emphasis will shift towards practice: you will have the opportunity to configure networks, analyze traffic, solve real-world problems, and reinforce your knowledge through hands-on exercises. Get ready—numerous labs and practical assignments are ahead, allowing you to confidently apply the theoretical knowledge in real scenarios.
You may be wondering: why not start the course with practical exercises right away? Why focus on theoretical foundations if practice is much more engaging? This approach might indeed seem more appealing at first glance.
However, in order to configure network equipment correctly, understand the underlying principles, and efficiently troubleshoot issues, a solid theoretical foundation is essential. Theory helps you understand why certain processes are implemented the way they are, how different layers of the network model interact, and what the implications of incorrect configurations might be. With this knowledge, you will be able to resolve real-world networking issues quickly and effectively.
As with many areas in computing, understanding networks requires studying a series of abstractions. The main set of abstractions in networking is the OSI model (Open Systems Interconnection), which helps to organize and structure the processes of data transmission between devices.
Before discussing the OSI model itself, it is important to introduce the concept of a “protocol.” In the context of networks, a protocol (more precisely, a data exchange protocol) is a set of rules that defines how devices communicate and exchange information. Different protocols are used for different purposes and services, optimizing network operations and ensuring device compatibility. You will learn about the most common protocols a bit later.
Let’s return to the OSI model. It consists of seven layers, each (except for the bottom — the physical layer) is an abstraction built on top of the previous one. This layered architecture is specifically designed to make it easier to understand network interactions, develop and debug software, and standardize the interoperability of equipment from different vendors. First, we will briefly review all the layers, and then we will take a closer look at some of the key ones.
+------------------+ +------------------+
| Computer 1 | | Computer 2 |
+------------------+ +------------------+
| Application | <-----> | Application |
+------------------+ +------------------+
| Presentation | <-----> | Presentation |
+------------------+ +------------------+
| Session | <-----> | Session |
+------------------+ +------------------+
| Transport | <-----> | Transport |
+------------------+ +------------------+
| Network | <-----> | Network |
+------------------+ +------------------+
| Data Link | <-----> | Data Link |
+------------------+ +------------------+
| Physical | <-----> | Physical |
+------------------+ +------------------+
| |
+-----------+-----------------+
|
Physical Medium
Each layer of the OSI model communicates directly only with its corresponding layer on another computer. This principle is known as “peer-to-peer communication.”
The “Physical Medium” refers to the cable, fiber optic, or wireless channel that actually transmits data bits between devices.
1. Physical Layer
This is the only layer of the OSI model that is not an abstraction. It deals with real physical devices and transmission media: copper and fiber optic cables, network adapters, antennas for wireless communication, optical transmitters and receivers, and other physical components.
At the physical layer, electrical or optical signals are transmitted to represent binary data (ones and zeros), clock pulses, various types of modulation, and signal levels. Everything related to the physical representation of data occurs at this layer.
The most well-known and widely used physical layer protocol is Ethernet. This layer also includes standards such as Wi-Fi, DSL, and fiber optic technologies. The correct selection and configuration of physical hardware is fundamental for the proper functioning of any network.
2. Data Link Layer
The data link layer bridges the gap between physical hardware and software logic—it stands “with one foot” in hardware and the other in abstraction. Unlike the physical layer, there is no need to deal with specific signal types or cables; at the data link layer, the focus is on the rules for organizing data transmission over the physical medium.
The main purpose of the data link layer is to ensure reliable and organized data transfer between devices within a local network or a single network segment. This layer defines the structure of frames, the organization of network packets, uses MAC addresses for device identification, and implements protocols for error control and media access management (such as CSMA/CD in Ethernet).
The data link layer abstracts away the physical characteristics and provides standard communication channels regardless of the hardware used. As a result, device interaction becomes predictable and reliable, independent of cable or adapter type.
3. Network Layer
At the network layer of the OSI model, the concept of IP addressing emerges. This layer is the realm of the IP protocol (Internet Protocol), which is responsible for addressing and routing data packets between different networks.
The main function of the network layer is to determine the route for transmitting packets from the sender to the receiver, even if the devices are in different local or global networks. It provides logical addressing (IP addresses), packet fragmentation and reassembly, route selection, and handling routing errors.
The transmission of network packets (datagrams) is assumed at this layer, and administrators can abstract away from the details of the data link layer. The network layer deals exclusively with routing and logical addressing, enabling the creation of scalable and flexible networks of any size.
Key protocols at this layer include: IPv4, IPv6, ICMP, routers, and dynamic routing protocols (such as OSPF and BGP).
4. Transport Layer
At the transport layer of the OSI model, routing is no longer a concern—the main focus is on the reliable or efficient delivery of data between applications running on different devices. Everything passed down to this layer by higher-level applications will be transmitted to the destination device according to the selected protocol’s properties.
There are two main protocols at the transport layer: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol):
Therefore, the choice between TCP and UDP depends on the application’s requirements for reliability and transmission speed.
5. Session Layer
The session layer of the OSI model is responsible for establishing, managing, and terminating sessions (logical connections) between applications. At this layer, the specifics of data transmission are abstracted away—the main task is to organize logical communication between two endpoints. A session is a sequence of interactions between applications or devices, often involving authentication, state management, and synchronization.
The session layer enables simultaneous connections, session recovery after failures, and the separation of data streams when multiple applications operate in parallel.
Examples of session layer protocols include L2TP (Layer 2 Tunneling Protocol), PPTP (Point-to-Point Tunneling Protocol), and NetBIOS. In modern systems, session management is often implemented at the transport or application layers (for instance, HTTP/HTTPS session support is handled at a higher level), but for VPN, tunneling, and certain enterprise solutions, the session layer still plays an important role.
6. Presentation Layer
The presentation layer is responsible for how data is formatted and represented during transmission between devices. While its functions are not always directly associated with networking, this layer plays a crucial role in ensuring compatibility between applications exchanging information.
At the presentation layer, data is converted between different formats and encodings, compressed for efficient transmission, and encrypted or decrypted for secure communication. This allows applications running on different platforms and operating systems to understand each other, regardless of how data is internally stored or processed.
Examples of presentation layer functions include: text encoding (ASCII, Unicode), multimedia data formats (JPEG, MPEG), data structure conversion (such as serialization), and the use of encryption protocols and standards (TLS/SSL).
7. Application Layer
The application layer is where user programs and services interact directly with the network. All well-known protocols operate at this level, providing access to network applications and services: email (SMTP, IMAP, POP3), web protocols (HTTP, HTTPS), messaging (IRC, XMPP), remote access (TELNET, SSH), telephony and video conferencing (SIP), and many others.
The application layer’s purpose is to provide an interface between the user, the application, and network services, as well as to handle, format, and transmit data across the network using the appropriate protocols. Authentication, authorization, file operations, message exchange, web requests, and other user-facing actions are all performed at this layer.
In the modern world, the application layer covers a wide range of network services, supporting email, websites, instant messaging, VoIP, cloud services, and numerous other Internet applications.
Let’s look at an example of decomposing the process of sending an email using the OSI model layers. Suppose you send an email to your friend:
In real life, there is rarely perfect isolation between OSI layers. During application development and operation, layers often overlap—many modern protocols implement functions from multiple layers at once to simplify or speed up data processing.
However, this kind of decomposition provides a clear understanding of how data exchange actually happens within a network. The OSI model allows a systematic approach to troubleshooting network issues: when a problem arises, identifying the affected layer helps narrow down potential causes and speeds up the resolution process.
The more precisely you determine which layer is experiencing the issue, the easier it becomes to automate diagnostics, configure monitoring, or even prevent problems before they impact users or services.
Next, we will take a closer look at the physical layer.