Computer Networking Explained: OSI 7-Layer Architecture, TCP/IP, and IPv4 Subnetting

Introduction to Computer Networking

A computer network is an interconnected collection of autonomous computing devices that exchange data and share hardware and software resources. In the G.C.E. A/L ICT curriculum, Unit 6 tests candidates on theoretical network models, practical hardware, and IP address management.

The 7 Layers of the OSI Reference Model

The Open Systems Interconnection (OSI) model standardized by ISO divides network communication into seven hierarchical layers:

1. Layer 7 - Application Layer: Interfaces directly with software applications (Protocols: HTTP, HTTPS, FTP, SMTP, DNS).
2. Layer 6 - Presentation Layer: Handles data translation, compression, and encryption/decryption (Formats: JPEG, ASCII, SSL/TLS).
3. Layer 5 - Session Layer: Establishes, manages, and terminates communication sessions between applications.
4. Layer 4 - Transport Layer: Provides end-to-end data delivery, flow control, and error recovery (Protocols: TCP for reliable delivery, UDP for low-latency streaming).
5. Layer 3 - Network Layer: Handles logical addressing (IPv4, IPv6) and path determination (Routing). Devices: Routers.
6. Layer 2 - Data Link Layer: Provides physical node-to-node addressing using MAC addresses and media access control. Devices: Switches, Bridges.
7. Layer 1 - Physical Layer: Transmits raw unstructured bitstreams over physical transmission media (Voltage levels, cables, radio waves). Devices: Hubs, Repeaters, Cables.

Transmission Media Comparison

Selecting the appropriate transmission medium depends on bandwidth requirements, distance, and environmental interference:

• Unshielded Twisted Pair (UTP): Cost-effective, easy to install, maximum length 100m without repeaters. Common in Local Area Networks (LANs).
• Fiber Optic Cable: Transmits light pulses through glass cores. Immune to Electromagnetic Interference (EMI), extremely high bandwidth, ideal for backbone and undersea networks.
• Wireless Transmission: Uses Radio Waves, Microwaves, or Infrared. Enables mobility at the cost of vulnerability to physical obstacles and signal attenuation.

Understanding IPv4 Addressing and Subnetting

An IPv4 address is a 32-bit numerical identifier divided into four 8-bit octets (e.g., 192.168.1.1).

IP Address Classes:

• Class A: Range 1.0.0.0 to 126.255.255.255 (Default mask: 255.0.0.0 / 8 bits network).
• Class B: Range 128.0.0.0 to 191.255.255.255 (Default mask: 255.255.0.0 / 16 bits network).
• Class C: Range 192.0.0.0 to 223.255.255.255 (Default mask: 255.255.255.0 / 24 bits network).

Subnetting Example:

Given the network 192.168.10.0/26:

• The subnet mask has 26 network bits: 255.255.255.192.
• Remaining host bits: 32 - 26 = 6 bits.
• Number of usable hosts per subnet: 2^6 - 2 = 62 hosts.
• Block size per subnet: 256 - 192 = 64.