Computer Networks

What is a Computer Network?

A computer network is a group of two or more computers connected together to share resources like files, printers, internet, and software. When you share a document between your laptop and phone via WiFi, you’re using a network.

Definition: A network is a collection of interconnected computers and devices that can communicate and share resources using a common set of rules (protocols).

flowchart LR
PAN["PAN<br/>~10m<br/>(Bluetooth)"] --> LAN["LAN<br/>Building<br/>(Ethernet/WiFi)"] --> MAN["MAN<br/>City<br/>(Cable TV)"] --> WAN["WAN<br/>Global<br/>(Internet)"]

Types of Networks by Size

LAN Features

• Covers a small area (single room, building, or campus)
• High data transfer speed (100 Mbps to 10 Gbps)
• Owned by a single organization
• Uses Ethernet cables or WiFi
• Standard PC hardware

WAN Features

• Covers large geographical areas (country, continent, globe)
• Lower speed than LAN (depends on connection)
• Uses telephone lines, fiber optic, satellites
• The Internet is the largest WAN in the world

Network Topologies

Topology is the physical or logical arrangement of computers in a network — how they are connected to each other.

Star Topology — all nodes connect to central hub:

flowchart TD
Hub((Hub/Switch))
PC1[PC 1] --- Hub
PC2[PC 2] --- Hub
PC3[PC 3] --- Hub
PC4[PC 4] --- Hub

Bus Topology — all nodes share one backbone cable:

flowchart LR
PC1[PC 1] --- Bus["═══ Backbone Cable ═══"]
PC2[PC 2] --- Bus
PC3[PC 3] --- Bus

Ring Topology — each node connects to exactly two others:

flowchart LR
PC1[PC 1] --> PC2[PC 2]
PC2 --> PC3[PC 3]
PC3 --> PC4[PC 4]
PC4 --> PC1

Mesh Topology — every node connects to every other node:

flowchart TD
A[PC 1] --- B[PC 2]
A --- C[PC 3]
A --- D[PC 4]
B --- C
B --- D
C --- D

Star Topology

• All computers connect to a central hub or switch
• If one computer fails, others continue working
• If the central hub fails, entire network goes down
• Most commonly used in offices and homes

Bus Topology

• All computers connect to a single backbone cable (bus)
• Data travels in both directions along the cable
• Cheap and easy to install
• If the main cable breaks, entire network fails
• Older technology, rarely used today

Ring Topology

• Each computer connects to exactly two others, forming a circle
• Data travels in one direction around the ring
• If one computer fails, the entire network may be disrupted
• Used in some industrial settings

Mesh Topology

• Every computer connects to every other computer
• Highly reliable — if one connection fails, data takes another path
• Expensive due to many cables
• Used in military and critical systems

Tree (Hierarchical) Topology

• Combination of star and bus topologies
• Computers organized in a hierarchy like branches of a tree
• Used in large organizations with departments

Network Devices

Key difference: Hub sends data to everyone (broadcast); Switch sends only to the destination (unicast). Switch is smarter and more efficient than Hub.

flowchart LR
subgraph "Your Home Network"
PC[Computer] --- Switch[Switch]
Phone[Phone] --- Switch
Switch --- Router[Router]
end
Router --- |"via ISP"| Internet((Internet))
Internet --- Server[Web Server]

Transmission Media (Network Cables)

Wired Media

Fiber optic cable — uses light pulses for ultra-fast data transfer (CC BY-SA, Wikimedia)

Fiber optic is the fastest — uses light instead of electricity. Immune to electromagnetic interference.

Wireless Media

Network Protocols — Quick Reference

A protocol is a set of rules that governs how data is transmitted between devices on a network. Key protocols used in networking:

For detailed coverage of HTTP, HTTPS, FTP, SMTP, OSI model, TCP vs UDP, and port numbers — see the Internet & Its Protocols lesson. For email protocols (SMTP, POP3, IMAP) — see the Email & Its Protocols lesson.

IP Address

An IP address (Internet Protocol address) is a unique numerical label assigned to every device on a network.

For IP classes (A-E), NAT, subnet masks, and IPv4 vs IPv6 in depth — see the Internet & Its Protocols lesson.

IPv4 addresses are running out globally, which is why IPv6 was introduced with a much larger address space.

Additional Network Types

Networking Standards (IEEE 802)

IEEE = Institute of Electrical and Electronics Engineers. The 802 family covers all LAN/MAN networking standards.

MAC Address

A MAC (Media Access Control) address is a 48-bit hardware address permanently assigned to each Network Interface Card (NIC).

• Format: AA:BB:CC:DD:EE:FF (6 pairs of hexadecimal digits)
• Unique to every network device in the world
• Works at Layer 2 (Data Link) of OSI model
• Also called physical address or hardware address
• ARP protocol maps IP addresses to MAC addresses

IP address can change (dynamic via DHCP), but MAC address is permanent (burned into hardware). However, MAC spoofing is possible via software.

Client-Server vs Peer-to-Peer Architecture

Bandwidth Units

Note: Internet speed is measured in bits per second (Mbps), while file sizes are in bytes (MB). 1 byte = 8 bits. So 100 Mbps speed downloads at ~12.5 MB/s.

Network Topology Comparison

LoRaWAN & Bluetooth Versions

LoRaWAN (Long Range Wide Area Network)

• Designed for IoT (Internet of Things) devices
• Low power consumption — battery can last years
• Long range — up to 15 km in rural areas
• Low data rate (suitable for sensors, not video)
• Used in smart agriculture, smart cities, industrial monitoring

Bluetooth Versions

Key Takeaways

• Network = computers connected to share resources; Internet = world’s largest WAN
• LAN (building), MAN (city), WAN (country/global), PAN (personal ~10m)
• CAN (Campus Area Network) = multiple LANs in a campus; SAN (Storage Area Network) = dedicated storage network
• Topologies: Star (most common), Bus (old), Ring, Mesh (most reliable, most expensive), Tree (hierarchy)
• Hub broadcasts to all; Switch sends to specific device (smart); Router connects different networks
• Fiber optic is fastest (uses light, up to 100 Gbps); Twisted pair (UTP) is most common for LAN
• IEEE 802.3 = Ethernet (wired LAN); IEEE 802.11 = WiFi (wireless LAN); IEEE 802.15 = Bluetooth/WPAN
• MAC address: 48-bit hardware address (AA:BB:CC:DD:EE:FF), permanent, Layer 2, mapped by ARP
• Client-Server = centralized (web hosting, banking); P2P = decentralized (BitTorrent, blockchain)
• Bandwidth: bps → Kbps → Mbps → Gbps → Tbps; Internet speed in bits, file size in bytes (1 byte = 8 bits)
• LoRaWAN: IoT, low power (years on battery), long range (up to 15 km), low data rate
• Bluetooth 5.3 = latest version; BLE introduced in 4.0; 5.0 = 2x speed, 4x range
• IPv4 (32-bit, 4.3B addresses, running out); IPv6 (128-bit, unlimited)
• TCP/IP is the foundation protocol of the internet

Summary Cheat Sheet