Computer Networks

9 min read

Computer networks connect autonomous computing devices so they can exchange data and share resources. From household Wi-Fi to global submarine cables that bring Pakistan its AAE-1 and SEA-ME-WE-5 capacity, networking is the backbone of the digital economy.

Computer Network

A collection of autonomous computers (or other devices) interconnected by communication links and protocols enabling data exchange and resource sharing.

Types of networks by scale

Type	Range	Example
PAN	Few metres	Bluetooth pairing
LAN	Building / campus	Office Ethernet, Wi-Fi
MAN	City	Metro Ethernet, cable TV
WAN	Country / world	The Internet
GAN	Global	Inmarsat, Iridium

Network topologies

Bus — single backbone; failure of cable disrupts all.
Star — central hub/switch; failure of hub disables network.
Ring — each device connected to two neighbours; token-based access.
Mesh — every device connected to every other; redundant, expensive.
Tree — hierarchical star-of-stars.
Hybrid — combinations.

The OSI reference model

Developed by ISO (1984), it has seven layers, top-down:

#	Layer	Function	Example protocols/units
7	Application	User services	HTTP, SMTP, FTP, DNS
6	Presentation	Encoding, encryption	TLS, JPEG, ASCII
5	Session	Dialog control	NetBIOS, RPC
4	Transport	End-to-end delivery	TCP, UDP, segments
3	Network	Routing, addressing	IP, ICMP, packets
2	Data Link	Frame delivery	Ethernet, PPP, frames
1	Physical	Bits over medium	Cables, wireless, signals

Mnemonic (top-down): All People Seem To Need Data Processing.

TCP/IP model

The Internet's de facto reference: four layers — Application, Transport, Internet, Network Access.

Layer mapping

Application ↔ OSI layers 5-7.
Transport ↔ OSI 4.
Internet ↔ OSI 3.
Network Access ↔ OSI 1-2.

Key Points

TCP is connection-oriented, reliable, ordered (uses 3-way handshake: SYN, SYN-ACK, ACK).
UDP is connectionless, unreliable, unordered — used for DNS, VoIP, video streaming, gaming.
IPv4 is 32 bits (~4.3 billion addresses); IPv6 is 128 bits (effectively unlimited).
The Internet is a "network of networks" interconnected by routers running BGP.

IP addressing

IPv4

32 bits in 4 dotted-decimal octets (e.g., 192.168.1.1).
Classes A, B, C, D (multicast), E (reserved).
CIDR notation replaces classful (e.g., 10.0.0.0/8).
Private ranges (RFC 1918): 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
Loopback: 127.0.0.0/8.

IPv6

128 bits in 8 groups of hexadecimal (e.g., 2001:0db8:85a3::8a2e:0370:7334).
Pakistan's IPv6 adoption is rising, driven by PTCL and mobile operators.

Subnetting

Splitting a network into smaller ones with a subnet mask. For /24, the first 24 bits are network, leaving 8 bits (254 usable) for hosts.

Important protocols

Application layer

HTTP/HTTPS — web (ports 80/443).
DNS — name resolution (port 53).
SMTP/POP3/IMAP — email (25/110/143; secure: 465/995/993).
FTP/SFTP — file transfer (21/22).
SSH — secure shell (22).
DHCP — automatic IP assignment.

Transport layer

TCP — congestion control, flow control, retransmission.
UDP — lightweight, no guarantees.
QUIC — UDP-based, used by HTTP/3.

Network layer

IP (v4 and v6).
ICMP — control and diagnostics (ping, traceroute).
ARP — IP-to-MAC resolution.
Routing protocols: RIP, OSPF, EIGRP, BGP.

Data link

Ethernet (IEEE 802.3).
Wi-Fi (IEEE 802.11) — variants a/b/g/n/ac/ax (Wi-Fi 6).
PPP for serial links.
VLAN (802.1Q) for logical segmentation.

Switching and routing

Hubs (Layer 1) — repeat signals; rarely used today.
Switches (Layer 2) — forward frames by MAC.
Routers (Layer 3) — forward packets by IP; run routing protocols.
Layer-3 switches combine both functions in enterprise LANs.

Routing algorithms

Distance-vector — RIP; based on hop count.
Link-state — OSPF; floods topology, runs Dijkstra.
Path-vector — BGP; policy-driven, used between ASes.

Network performance

Key metrics:

Bandwidth — capacity (bps).
Throughput — actual rate achieved.
Latency — delay (ms).
Jitter — variation in delay.
Packet loss — discarded packets (%).
MTU — Maximum Transmission Unit (typically 1500 bytes for Ethernet).

Wireless and mobile

Wi-Fi 6 (802.11ax) — higher throughput, OFDMA, MU-MIMO.
Cellular generations: 1G (analog), 2G (GSM), 3G (UMTS), 4G (LTE), 5G (mmWave, sub-6 GHz). Pakistan launched 5G trials but is yet to issue commercial 5G spectrum (status as of 2025).
Pakistan's mobile operators: Jazz, Zong, Telenor, Ufone-PTCL. Combined teledensity exceeds 85%.

Network security

Confidentiality — keep data secret (encryption).
Integrity — prevent tampering (hashes, MAC).
Availability — resist DoS/DDoS.
Authentication — verify identity.
Non-repudiation — sender cannot deny.

Common attacks

DoS / DDoS — flood resources.
MITM — intercept communication.
ARP spoofing, DNS poisoning.
SQL injection, XSS, CSRF (web).
Phishing, ransomware.

Defences

Firewalls — packet filtering, stateful, application-layer.
IDS/IPS — intrusion detection/prevention.
VPN — secure tunnels (IPsec, SSL/TLS).
TLS — secures HTTP, SMTP, IMAP, etc.
PKI — public-key infrastructure with X.509 certificates.

For CSS short-answer questions, learn the TCP three-way handshake in detail: SYN (client → server), SYN-ACK (server → client), ACK (client → server). Examiners frequently ask candidates to draw a diagram and identify why the third ACK is necessary.

Cloud and the future

Cloud computing models: IaaS, PaaS, SaaS; major providers AWS, Azure, Google Cloud.
SDN (Software-Defined Networking) — programmable control plane.
NFV (Network Function Virtualisation) — virtualised firewalls, routers.
6G research — terahertz frequencies, AI-native networking.
Pakistan's MoITT Cloud-First Policy mandates public-sector cloud adoption.

A confident grasp of OSI vs. TCP/IP, IP addressing, common protocols and security threats is what distinguishes a serious CSS aspirant from a candidate who has merely memorised acronyms.