AQA 7517 · Paper 2 · 4.9.3a

Packet Switching,
IP & DNS

How data travels across the internet — packets, routers, IP addressing and DNS

WHAT YOU'LL LEARN

Packet switching · IP addresses · DNS · Routers · Protocols

AQA SPEC LINK

4.9.3 — The internet: structure, packet switching, IP, DNS, routers

Packet Switching

How Data is Sent: Packet Switching

Data is broken into small packets. Each packet travels independently across the network — possibly via different routes — and is reassembled at the destination.

Each packet contains: header (source IP, destination IP, packet number, TTL) + payload (data) + trailer (error checking)

Packets may arrive out of order — reassembled using packet numbers

If a packet is lost or corrupted, only that packet is re-sent — not the whole message

Advantage: efficient use of network bandwidth; multiple users share the same physical network

IP Addresses

IPv4

32-bit address — 4 groups of 8 bits (dotted decimal notation) e.g. 192.168.1.1. Gives ~4.3 billion unique addresses — nearly exhausted.

IPv6

128-bit address — 8 groups of 4 hex digits e.g. 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Gives 3.4 × 10³⁸ unique addresses — solves exhaustion problem.

DNS

Domain Name System (DNS)

DNS is the internet's phonebook. It translates human-readable domain names (e.g. cszone.co.uk) into IP addresses (e.g. 104.21.5.82) so computers can route packets correctly.

Browser sends DNS query → DNS resolver checks its cache → if not found, queries root nameserver → TLD nameserver (.co.uk) → authoritative nameserver

Result is cached locally (TTL set by domain owner) to speed up future lookups

Without DNS you would need to memorise IP addresses for every website

Routers

Routers & Routing

Router — device that forwards packets between networks using IP addresses

Each router examines the destination IP in the packet header and uses its routing table to decide which link to forward the packet on

Routing table: mapping of destination networks to next-hop routers or outgoing interfaces

If one route is down, routers dynamically find alternative paths (resilience)

TTL (Time To Live) — decremented by 1 at each hop; packet discarded when TTL = 0 (prevents infinite loops)

Home router connects local network (LAN) to ISP (WAN) and performs NAT

URL vs IP

URL, Domain & IP Structure

https://www.cszone.co.uk/lessons
https://
www
cszone.co.uk
/lessons
Protocol
Subdomain
Domain + TLD
Path

DNS resolves cszone.co.uk → IP address; browser then makes HTTP request to that IP

TLD (Top-Level Domain): .co.uk = UK commercial; .com = global commercial; .edu = education; .gov = government

Circuit vs Packet Switching

Circuit Switching vs Packet Switching

Feature	Circuit Switching	Packet Switching
Dedicated path?	Yes	No — dynamic routing
Efficiency	Poor (idle capacity wasted)	High (bandwidth shared)
Used for	Old telephone networks	Internet
Fault tolerance	Low (single path)	High (reroutable)

Packet Structure

Anatomy of a Packet

HEADER

Source IP
Destination IP
Packet no.
TTL
Protocol

PAYLOAD

Actual data being
transmitted
(fragment of the
original message)

TRAILER

Error checking
(checksum/CRC)

CRC (Cyclic Redundancy Check): mathematical value computed from payload; receiver recalculates and compares to detect corruption

AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style

(a) Describe how packet switching works when sending a file across the internet. [4]
(b) Explain the purpose of DNS. [2]
(c) State TWO pieces of information found in a packet header. [2]

[8 marks]

4 marks

(a) File is broken into packets [1]; each packet contains source/destination IP, packet number, payload and error-checking data [1]; packets travel independently via different routes through routers [1]; packets are reassembled in correct order at destination using packet numbers [1]

2 marks

(b) DNS translates/resolves human-readable domain names (e.g. cszone.co.uk) [1] into IP addresses so computers can route data to the correct destination [1]

2 marks

(c) Any 2 from: source IP address / destination IP address / packet number / TTL (Time To Live) / protocol type

Summary

Key Points to Remember

Packet switching — data split into packets, travel independently, reassembled at destination

Packet = header + payload + trailer; header has src/dest IP, packet no., TTL

IP address — IPv4 (32-bit, 4.3B addresses); IPv6 (128-bit, solves exhaustion)

DNS — translates domain names to IP addresses; cached for speed

Routers — forward packets using routing tables; TTL prevents infinite loops

🎉 Lesson complete — move to the quiz!