HKDSE ICT | Networking and Data Transmission Prep Guide (for tutors)

HKDSE ICT | Networking and Data Transmission Prep Guide (for tutors) | Bandwidth, Subnets and Latency

2026-11-01

Tutor lesson-prep guides · Secondary ICT

Students say the internet is slow, then start multiplying when asked how long 750 MB takes. This lesson separates bit from byte, then counts hosts from the prefix length instead of guessing.

Objectives — Students can: (1) compute transfer time from bandwidth and file size, handling bit and byte correctly; (2) work out subnet size and usable hosts from a prefix; (3) separate bandwidth from latency and name the bottleneck stage.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Bit and byte	How many bits in a byte	eight
Powers of two	What 2^6 is	64
Prefixes	Ratio of M to k in networking	1,000, decimal

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Bandwidth	bps versus B/s	convert once themselves
10-26 min	Transfer time	t = size x 8 / bandwidth	get 60 s
26-42 min	Subnetting	prefix fixes the address count	/26 gives 62 usable
42-52 min	Latency and bottlenecks	big bandwidth is not fast response	name the slowest stage
52-56 min	Answer format	formula, unit, conclusion	rewrite one answer
56-60 min	Exit check	three questions marked on the spot	done unaided

03 Teaching script (read it out)

Scene 1: Bandwidth is bps, not B/s

100 Mbps means 100 million bits per second. A 750 MB file is 6000 Mbit, so t = 6000 / 100 = 60 s. Convert everything to bits first; getting this wrong ruins the rest.

Task: how long does the same file take at 40 Mbps upload (150 s)?

Scene 2: Subnets: count addresses, not digits

/26 means 26 prefix bits, leaving 6 host bits, so 2^6 = 64 addresses. Remove the network and broadcast addresses and 62 remain usable. 50 machines fit in /26; /27 with 30 usable would not.

Reminder: usable hosts is always 2^n minus 2, never just 64.

Scene 3: Latency has four parts

Latency = processing + queueing + transmission + propagation. Across 4 routers at 2 ms each, one way adds 8 ms. Bandwidth decides how much moves, latency decides when it starts, so gaming wants low latency and downloads want bandwidth.

Task: explain why video calls care about latency far more than bandwidth.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Download time	MB x 8 / Mbps	60 s
Calculation	Usable hosts	2^(32-prefix) – 2	62 hosts
Selection	Is the prefix big enough	usable hosts >= requirement	50 machines need /26
Explanation	Bandwidth versus latency	throughput versus response	not interchangeable

Sanity check (teach this): Transfer time must be positive and sensible: 750 MB at 100 Mbps is about a minute, so eight minutes means the factor of eight was missed. Usable hosts is always a power of two minus two.

05 Independent practice (3 questions)

Question	Answer	What to watch
750 MB at 100 Mbps	60 s	convert to bits first
Usable hosts in /26	62 hosts	2^6 minus 2
Which metric affects a video call	latency	bandwidth only changes quality

06 Common mistakes and what to say

Mistake	Student writes	Say this
Byte treated as bit	answering 7.5 s	“MB must be multiplied by eight first.”
Forgetting to remove two addresses	answering 64	“Network and broadcast addresses cannot be assigned.”
Bandwidth used for latency	saying more bandwidth lowers latency	“Bandwidth moves volume, latency is arrival time.”
Bigger prefix, more hosts	claiming /27 beats /26	“A bigger prefix leaves fewer host bits.”

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — all chapters
HKDSE ICT practice paper — Section B networking questions

08 Exit check (last 5 minutes)

Question	Answer	Decision
750 MB at 100 Mbps	60 s	three correct moves on to security
Usable hosts in /26	62 hosts	one wrong means redo bit and byte
Bandwidth versus latency	throughput versus response	two wrong means repeat act three

09 Tutor FAQ

Must students memorise every prefix?

Remember /24 gives 254 usable and each extra prefix bit halves it; the rest follows.

Why decimal prefixes in networking?

Bandwidth is quoted in powers of ten while storage is often powers of two, so follow the units given.

Why does IPv6 have so many addresses?

It moves from 32 to 128 bits, so every device can have a globally unique address without NAT.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.