English:Computer Networks
Computer Networks
Introduction
Computer Networks is an aiMOOC for learners in Grades 11–13 who want to understand how digital devices exchange data. You will connect ideas from Computer science, Information technology, and Cybersecurity with practical network design, packet analysis, troubleshooting, and responsible system administration.
A computer network is a set of connected computing devices that exchange data and share resources by following agreed communication rules called protocols. Networks range from a few devices in one room to the global Internet. Modern networking depends on packet switching, layered protocols, addressing, routing, switching, wired and wireless media, and security controls.
Before you begin, think about your own school, home, or workplace network. Which devices are endpoints? Which devices connect networks? Which links are wired or wireless? Which services would stop working if DNS failed?
Learning Goals
By the end of this aiMOOC, you should be able to explain how data moves through a network; distinguish LANs, WANs, switches, routers, access points, and firewalls; compare physical and logical topologies; describe packet switching and encapsulation; interpret IPv4 addresses and subnet prefixes; distinguish TCP from UDP; relate the TCP/IP suite to the OSI model; evaluate bandwidth, throughput, latency, jitter, and packet loss; apply basic network security principles; and troubleshoot a simple network systematically and ethically.
Network Scope and Architecture
LAN, WAN, and Other Network Scales
A local area network or LAN connects devices within a limited area such as a classroom, office, laboratory, or building. Ethernet and Wi-Fi are common LAN technologies. A wide area network or WAN connects networks across larger geographic areas. The Internet is a network of networks rather than one single LAN or one single device.
A PAN connects devices around one person, while a MAN spans a city-scale area. These labels describe scope rather than a single protocol. A school may have several LANs, perhaps separated by VLANs, connected by routers to a service-provider WAN.
Networks can also be described by how applications communicate. In a client-server design, clients request services from servers. In a peer-to-peer design, participating devices can provide and consume resources more directly. Real systems can combine both approaches.
Physical Media
Network data must cross a transmission medium. Copper twisted-pair cable carries electrical signals, optical fiber carries light, and wireless networks use radio waves. Each medium has trade-offs in cost, distance, bandwidth, interference resistance, and installation requirements.
Copper Ethernet is common for endpoint connections inside buildings. Fiber is valuable for higher-capacity links, longer distances, and environments where immunity to electromagnetic interference matters. Wi-Fi enables mobility and convenient access, but shared radio spectrum, obstacles, interference, and distance can affect performance.
Topologies and Network Devices
Network Topologies
A topology describes how network nodes and links are arranged. A physical topology describes the actual cabling or radio relationships. A logical topology describes how data is organized and forwarded.
In a star topology, devices connect to a central device such as a switch. This is common in modern switched Ethernet LANs. A mesh topology provides multiple paths between nodes and can improve resilience, although it costs more to build and manage. Bus and ring topologies are important historically and still help you compare design trade-offs.
When you evaluate a topology, ask what happens if one link fails, what happens if the central device fails, how easily the network can grow, and how much cabling or radio capacity is required.
Switches, Routers, Access Points, Modems, and Firewalls
A switch commonly forwards Ethernet frames inside a LAN. It learns which MAC addresses are reachable through which ports and uses that information to make forwarding decisions. A router forwards IP packets between different networks or subnets by using a routing table.
A wireless access point connects Wi-Fi devices to a network. A modem or other access device adapts digital traffic to the signaling method used by an access connection. A firewall enforces policy by allowing, rejecting, or inspecting traffic according to configured rules. Home gateways often combine several of these functions in one device.
A useful distinction is this: switches usually make local forwarding decisions using link-layer information, while routers make inter-network forwarding decisions using network-layer addresses. In real equipment the boundary can blur because multilayer switches can also route.
How Data Moves Through a Network
Packet Switching
Most modern computer networks move data in formatted units called packets or, at particular layers, frames, segments, or datagrams. Packet switching lets many users share communication links. If one sender is temporarily silent, other traffic can use the available capacity.
A packet contains control information and payload. Headers can identify source and destination addresses, protocol information, sequence information, and other fields needed for delivery. A receiving system removes headers as data moves upward through its protocol stack.
A long message is not simply placed on the network as one unlimited block. Protocols divide data into manageable units, and each link has a maximum transmission unit that limits the size of a network-layer packet it can carry without special handling.
A Web Request as a Packet Journey
Suppose you enter a website name in a browser. First, your device usually needs an IP address for that name, so it performs a DNS lookup. The application then creates traffic for the destination service. The transport layer manages communication between processes. IP provides network-layer addressing and forwarding. Ethernet or Wi-Fi carries frames across each local link.
At a routed hop, the link-layer frame is replaced for the next link, while the IP packet continues toward its destination. The router reduces the IPv4 TTL or IPv6 Hop Limit so that packets cannot circulate forever. If network address translation is used, an address or port can also be changed at the translation boundary.
The reply follows the same general layered logic in the opposite direction, although the exact route does not have to be identical.
Addressing, Subnetting, and Names
MAC and IP Addresses
A MAC address identifies a network interface at the data-link level within technologies such as Ethernet. An IP address identifies an interface at the Internet layer and supports routing across networks. These address types solve different problems and should not be treated as interchangeable.
IPv4 addresses are 32 bits long. They are commonly written as four decimal octets, for example 192.168.10.25. IPv6 addresses are 128 bits long and are written in hexadecimal groups separated by colons. The much larger IPv6 address space supports far more unique addresses and new addressing practices.
Subnets and Prefixes
A subnet divides an IP network into smaller logical networks. Classless Inter-Domain Routing notation writes a prefix length after an address. In the IPv4 network 192.168.10.0/24, the prefix length means that 24 bits identify the network portion. This leaves 8 bits for addresses within that subnet, giving 256 total address values. In a conventional IPv4 LAN, the all-zero host value represents the network address and the all-one host value represents the broadcast address, leaving 254 addresses for ordinary host assignments.
Subnetting helps organize networks, control broadcast domains, apply policy, and route traffic efficiently. You should choose subnets based on real requirements rather than merely memorizing address patterns.
DNS and Ports
Humans usually prefer names such as example.org, while routers forward traffic using IP addresses. DNS is a distributed naming system that maps names to information such as IP addresses. Traditional DNS commonly uses UDP or TCP port 53, while modern encrypted DNS mechanisms can use other transports.
At the transport layer, TCP and UDP use port numbers to help deliver data to the correct application process. A server can provide multiple services on one IP address because different services can listen on different ports. Ports identify software endpoints; they are not physical sockets on a network switch.
Layered Models and Protocols
Why Layers Matter
Layering divides a complicated communication problem into smaller functions. A layer provides services to the layer above and uses services from the layer below. This modularity helps engineers develop, troubleshoot, and replace technologies without redesigning every other part of the system.
Encapsulation is the process of adding protocol information as data moves down the stack. Decapsulation removes that information as data moves up the receiving stack.
The OSI Model
The OSI model is a conceptual seven-layer reference model. From top to bottom, the layers are Application, Presentation, Session, Transport, Network, Data Link, and Physical. The model is useful for vocabulary and troubleshooting, but real Internet protocols do not always fit into seven perfectly separated boxes.
A troubleshooting question such as “Is the cable connected?” focuses on the Physical layer. “Is the Ethernet frame reaching the correct interface?” focuses on Data Link behavior. “Does the host have the correct IP route?” focuses on the Network layer. “Is the required TCP or UDP service reachable?” focuses on the Transport layer.
The TCP/IP Model
The TCP/IP model is closely connected to the protocols used on the Internet. A common four-layer view is Application, Transport, Internet, and Link. The mapping to OSI is approximate rather than one-to-one.
At the Application layer, protocols such as HTTP, DNS, and SMTP support user-facing services. At the Transport layer, TCP provides a reliable ordered byte stream, while UDP provides connectionless datagrams with lower protocol overhead and no built-in guarantee of delivery or ordering. At the Internet layer, IP provides addressing and forwarding. At the Link layer, technologies such as Ethernet and Wi-Fi move data across a local link.
TCP, UDP, and ICMP
TCP establishes state between endpoints and uses sequence numbers, acknowledgements, retransmission, flow control, and congestion-control mechanisms to provide reliable ordered delivery of a byte stream. These features are useful for many web, file-transfer, and remote-access applications.
UDP sends independent datagrams without creating TCP-style connection state or guaranteeing delivery and order. Applications that use UDP must decide which reliability features they need. Real-time audio, gaming, DNS, and modern transport protocols can use UDP because low delay, application-controlled recovery, or protocol flexibility may matter.
ICMP carries control and diagnostic messages for IP. Tools such as ping use ICMP echo messages on many systems, while traceroute-style tools use responses triggered by changing hop limits or TTL values. Network policies can filter diagnostic traffic, so a failed ping does not automatically prove that a host or service is offline.
Performance and Quality of Service
Bandwidth is the theoretical or configured capacity of a link, usually measured in bits per second. Throughput is the rate of useful data actually transferred. Latency is the time taken for data to travel from source to destination. Jitter describes variation in delay. Packet loss is the fraction of packets that fail to arrive.
These metrics affect applications differently. A large file transfer benefits strongly from high throughput. Interactive voice and video are sensitive to latency, jitter, and loss. A network can have high bandwidth and still feel slow if congestion, poor Wi-Fi conditions, long physical paths, overloaded devices, or application delays increase response time.
Quality of Service mechanisms can classify and prioritize traffic, but they do not create unlimited capacity. Good design still requires appropriate bandwidth, low-loss links, sensible routing, and capacity planning.
Network Security
A secure network aims to protect confidentiality, integrity, and availability. Security is not one product; it is a set of technical and organizational controls.
Encryption such as TLS protects data in transit against passive reading and helps authenticate servers when certificates are validated correctly. Secure Wi-Fi configurations use modern protection such as WPA2 or WPA3 rather than obsolete mechanisms. Firewalls restrict unwanted traffic. Network segmentation limits which systems can communicate directly. Patching reduces exposure to known vulnerabilities. Strong authentication and least privilege reduce the damage that compromised accounts can cause.
Monitoring matters as well. Logs, intrusion-detection systems, traffic baselines, and alerting can reveal abnormal behavior. However, monitoring must respect law, policy, and privacy. Only capture or inspect traffic on systems and networks you are authorized to analyze.
Security also depends on people. Clear procedures, backups, incident-response plans, and awareness of social engineering are part of resilient networking.
Systematic Troubleshooting
Troubleshooting works best when you test one layer or assumption at a time instead of changing many settings at once.
Start with the physical and link conditions: power, cables, link indicators, Wi-Fi association, and interface state. Then check configuration: IP address, subnet prefix, default gateway, and DNS settings. Next test local reachability, then the gateway, then a remote IP address. If IP connectivity works but names fail, investigate DNS. If the host and DNS work but one application fails, investigate the service, port, firewall policy, certificate, or application configuration.
Useful tools include ping for reachability and round-trip observations, traceroute or tracert for path investigation, ipconfig or ip for interface information, and nslookup or dig for DNS questions. Packet analyzers such as Wireshark can show protocol details in an authorized lab environment.
Record what you tested, what you expected, what actually happened, and what changed. This turns troubleshooting into evidence-based reasoning rather than guesswork.
Global Infrastructure and Careers
The Internet depends on local networks, Internet service providers, exchange points, data centers, terrestrial fiber, radio links, and submarine communication cables. The following Wikimedia Commons map is a historical 2015 snapshot, so use it to study the idea of global cable routes rather than as a current operational map.
Network knowledge supports careers such as network technician, systems administrator, network engineer, cloud engineer, security analyst, telecommunications specialist, and IT support professional. These roles combine technical knowledge with documentation, communication, risk management, and continuous learning because standards and products evolve.
Interactive Tasks
Quiz: Test Your Knowledge
Which device primarily forwards IP packets between different networks? (Router) (!Switch) (!Access point) (!Repeater)
Which metric describes variation in packet delay? (Jitter) (!Bandwidth) (!Addressing) (!Encryption)
Which protocol provides a reliable ordered byte stream? (TCP) (!UDP) (!IP) (!Ethernet)
What does a subnet prefix identify? (Network portion of an IP address) (!Physical cable category) (!Application password) (!Wireless channel name)
Which technology is commonly used for wired local area networks? (Ethernet) (!DNS) (!SMTP) (!HTML)
Which system translates domain names into information such as IP addresses? (DNS) (!TCP) (!VLAN) (!USB)
Which OSI layer is most directly concerned with IP routing? (Network layer) (!Presentation layer) (!Session layer) (!Physical layer)
What is the main purpose of a firewall? (Enforce network traffic policy) (!Increase monitor resolution) (!Store all user files) (!Convert text into images)
Which value is 128 bits long? (IPv6 address) (!Ethernet port number) (!DNS label) (!TCP flag)
What should you check when IP addresses work but domain names fail? (DNS configuration) (!Keyboard layout) (!Screen brightness) (!Printer paper)
Memory Game
| Router | Forwards traffic between distinct IP networks |
| Switch | Directs local Ethernet frames toward the appropriate port |
| DNS | Resolves human-readable domain names into network information |
| Subnet | Divides an address space into a smaller logical network |
| Ethernet | Family of widely used wired LAN technologies |
| Firewall | Applies rules that permit or block traffic |
| Latency | Time delay between sending data and receiving a response |
| Jitter | Variation in network delay over time |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Forward frames inside a LAN | Switch |
| Forward packets between networks | Router |
| Resolve a host name | DNS |
| Protect data in transit | TLS |
| Measure path delay variation | Jitter |
Match each network function to the technology or concept that performs it most directly.
Crossword Puzzle
| Router | Which device forwards IP packets between networks? |
| Ethernet | Which wired LAN technology is standardized in the IEEE 802 family? |
| Packet | What formatted unit carries data across a packet-switched network? |
| Subnet | What smaller logical network is created from an IP address block? |
| Protocol | What agreed set of rules governs communication? |
| Firewall | What security control filters traffic according to policy? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Home network map: Draw a labelled diagram of a safe example home or classroom network showing endpoints, a switch or access point, a router, and the Internet connection.
- Network vocabulary guide: Create a one-page illustrated glossary for ten core terms such as packet, protocol, router, switch, DNS, subnet, latency, and firewall.
- Cable and wireless comparison: Photograph or sketch examples of wired and wireless connections you are allowed to document, then explain one advantage and one limitation of each medium.
- Packet journey storyboard: Produce a six-panel storyboard showing what happens when you enter a website name, from DNS lookup to the returned web data.
Standard
- Latency experiment: Measure round-trip times to several authorized public or school-approved destinations at different times, record your method, and explain possible causes of variation.
- Network professional interview: Interview an IT technician, network administrator, or teacher about common network faults, documentation habits, security responsibilities, and useful career skills.
- Wireshark protocol observation: In a teacher-approved lab, capture only traffic you are authorized to inspect, identify several protocols, and annotate a screenshot without exposing private credentials or personal data.
- Subnet design project: Design an IPv4 addressing plan for a fictional school with separate student, staff, laboratory, and server networks, and justify the chosen prefix sizes.
Advanced
- Redundant network design: Design two versions of a small campus network, one low-cost and one high-resilience, then compare single points of failure, capacity, security zones, and recovery options.
- TCP and UDP investigation: Create a short technical report comparing how two approved applications use transport protocols, including reliability needs, latency sensitivity, and packet-capture evidence from a controlled lab.
- Network security audit simulation: Evaluate a fictional network diagram against a checklist covering segmentation, patching, authentication, encryption, firewall placement, logging, and backup connectivity, then prioritize improvements.
- Troubleshooting video: Produce a short instructional video that demonstrates a teacher-created network fault, shows a layer-by-layer diagnostic process, and explains why each test confirms or rejects a hypothesis.
Learning Assessment
- Network design reasoning: Given a floor plan and user requirements, design a LAN and justify device placement, media choices, topology, addressing, and one resilience measure.
- Packet path analysis: Explain how a web request travels from a client to a remote server, identifying the roles of DNS, TCP or UDP, IP, Ethernet or Wi-Fi, switches, and routers.
- Subnet transfer task: Apply CIDR reasoning to an unfamiliar IPv4 address block, determine a suitable subdivision for several departments, and explain trade-offs between address efficiency and future growth.
- Performance diagnosis: Compare evidence from throughput, latency, jitter, and loss measurements and decide which metric best explains three different application problems.
- Security architecture: Propose layered controls for a school network and explain how segmentation, encryption, authentication, firewalling, patching, monitoring, and backups reduce different risks.
- Fault isolation challenge: Use a provided set of symptoms and test results to locate a likely failure at the physical, link, network, transport, naming, or application level and justify the next diagnostic step.
Evidence of Learning
Strong evidence of learning should show more than memorized definitions. It should demonstrate that you can connect concepts, apply methods, communicate decisions, and transfer your knowledge to new network situations.
| Evidence area | What successful learning can look like |
|---|---|
| Knowledge | Accurate explanations of addressing, packet switching, topologies, layered models, transport protocols, naming, routing, switching, performance, and security. |
| Practical skills | Safe use of diagnostic tools, interpretation of interface settings and packet traces, subnet calculations, topology analysis, and structured fault isolation. |
| Products | Clear network diagrams, addressing plans, experiment records, protocol annotations, security reviews, reports, presentations, and instructional media. |
| Reasoning | Justified choices that connect requirements to bandwidth, resilience, topology, addressing, security controls, cost, and maintainability. |
| Transfer | Ability to analyze an unfamiliar network scenario, identify relevant layers and protocols, select useful evidence, and propose defensible improvements. |
| Professional practice | Ethical handling of network data, respect for authorization and privacy, careful documentation, precise technical language, and constructive teamwork. |
OERs on the Topic
The embedded English Wikipedia article provides a broad reference overview. Use it together with the Wikimedia Commons media in this course and, for formal Internet standards, consult the publicly accessible IETF and RFC Editor documentation.
Linked Learning Areas
The essential ideas of computer networking connect hardware, protocols, addressing, routing, performance, security, troubleshooting, and the human work of designing and operating reliable systems.
