DSE ICT | Networks and Cybersecurity Tutor Prep Guide | 20 seconds, 62 addresses, 47.6 bits

2026-11-03

Tutor lesson-prep guides · Secondary ICT

Students treat Mbps as megabytes, read a slash twenty-six as 26 addresses, and think a symbol makes a password safe. This lesson replaces all three with arithmetic: 2,000 Mb over 100 Mbps is 20 seconds, a slash twenty-six gives 64 addresses and 62 usable, and eight characters give about 47.6 bits.

Objectives — Students can: (1) convert a file size to bits and divide by bandwidth (250 MB over 100 Mbps gives 20 seconds ideal) and explain why real transfers are slower; (2) derive 64 addresses, 62 usable hosts and the mask 255.255.255.192 from a slash twenty-six; (3) compute about 47.6 bits for an eight-character password and name three attacks with a defence each.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Bits and bytes	How many bits are in a byte?	8 bits
Powers of two	What is two to the power six?	64
Logarithms	Two to the eighth is 256, so what is log base two of 256?	8
Prefixes	Mega means ten to the power what?	Six, and bandwidth uses decimal units

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Bandwidth units	Mbps against MB	Convert 250 MB to bits
10-22 min	Real throughput	20 seconds ideal, 70 per cent efficiency	Compute about 29 seconds and say why
22-36 min	Subnetting	Addresses, usable hosts, mask	Write 255.255.255.192
36-46 min	Password strength	62 symbols, eight positions	Compute about 47.6 bits
46-54 min	Attacks and defences	Phishing, interception, brute force	Pair each attack with a defence
54-60 min	Exit check	Three written questions marked live	Work independently

03 Teaching script (read it out)

Scene 1: Mbps counts bits per second, not bytes

Convert 250 MB to bits first: 250 times 8 gives 2000 Mb. On a 100 Mbps link the ideal time is 2000 divided by 100, which is 20 seconds. Real transfers are often thirty per cent slower because the quoted rate is a ceiling, headers and acknowledgements take a share, and wireless links retransmit. Make the multiplication by eight the first line of every answer and the unit error disappears.

Ask how many megabits a 250 MB file is, and why it is not 250.

Scene 2: Subnetting: count addresses, then subtract two

A slash 26 means the first 26 bits name the network, leaving 6 bits for hosts, so the address count is two to the power 6, which is 64. The first address names the network and the last is broadcast, leaving 62 usable hosts. The final mask octet is 256 minus 64, which is 192. Teach the order: power of two, subtract two, then write the mask.

Ask for the address count, the usable count and the mask for a slash twenty-six.

Scene 3: Password strength is a combination count

Fifty-two letters plus ten digits gives 62 symbols; choosing 8 of them gives 62 to the power 8, about 2.18e+14 combinations, or roughly 47.6 bits. Length beats symbols: one extra character multiplies the space by 62, about six bits, while swapping a letter for a symbol enlarges the set by only a few. Pair that with login throttling and two-factor authentication for a real defence.

Ask why adding length beats adding symbols.

04 Question types and worked steps

Type	Example	Steps	Answer
Transfer time	250 MB over 100 Mbps	Multiply by eight, divide by bandwidth, divide by efficiency if given	20 seconds ideal, about 29 seconds real
Subnet maths	A slash twenty-six	Two to the host bits, subtract two, mask is 256 minus the block	64 addresses, 62 usable, mask 255.255.255.192
Password entropy	62 symbols, eight positions	Size of the symbol set, raise to the length, take log base two	About 47.6 bits
Latency and parallelism	20 requests, 40 ms round trip, 6 connections	Serial total, divide by parallelism and round up, multiply by the round trip	0.80 seconds against 0.16 seconds

Sanity check (teach this): A check on direction: the bit count 2000 Mb is larger than the megabyte figure because of the factor of eight, and dividing by 100 gives 20 seconds, the right order for a home connection. A slash 26 block of 64 addresses sounds generous until the network and broadcast addresses are removed, leaving 62, which matches the familiar rule of two to the n minus two. At 47.6 bits an eight-character password beats old 40-bit encryption but trails 128-bit, which is exactly why passphrases win.

05 Independent practice (3 questions)

Question	Answer	What to watch
Ideal time for 250 MB over 100 Mbps	20 seconds	Did you forget the factor of eight
Usable hosts in a slash twenty-six	62	Did you forget to subtract two
Entropy of eight characters from 62 symbols	About 47.6 bits	Did you use 26 symbols
Total time for 20 requests, 6 parallel, 40 ms round trip	About 0.16 seconds	Did you compute the serial 0.8 seconds

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating Mbps as megabytes	100 Mbps means 100 MB per second, so 2.5 seconds	Mbps counts bits and files are bytes, so multiply by eight first; your answer is eight times too small.
Forgetting the two reserved addresses	A slash twenty-six gives 64 usable addresses	The first and last cannot host a device, so 64 minus 2 leaves 62.
Judging passwords by symbols alone	Adding an exclamation mark makes it safe	The combination count decides; one extra character multiplies by 62 while one symbol adds a handful.
Reading HTTPS as proof of trust	A padlock means the site is genuine	HTTPS only encrypts the connection; phishing sites have padlocks too, so check the domain and certificate.

07 Homework (paper, section and question numbers)

ICT chapter index — Revise cloud and IoT, then do this week network questions
Cloud computing and IoT — Use cloud services to separate bandwidth from latency
Programming: arrays and functions — Write a loop that converts many file sizes to transfer times

08 Exit check (last 5 minutes)

Question	Answer	Decision
Ideal seconds for 250 MB over 100 Mbps	20 seconds	Error: no factor of eight
Usable hosts in a slash twenty-six	62	Error: no subtraction
Entropy of eight characters from 62 symbols	47.6 bits	Error: used 26 symbols
What HTTPS guarantees	An encrypted connection, not a trustworthy site	Error: treated as a safety certificate

09 Tutor FAQ

How far must powers of two be memorised?

Up to two to the twelfth, which is 4096, covers DSE subnetting, and the rest can be multiplied on the spot.

Why is wireless so much slower than wired?

A wireless channel is shared, weakened by walls and distance, and retransmits lost frames, so real throughput is often half the quoted rate.

Is two-factor authentication always safe?

It limits the damage of a stolen password, but phishing can relay a one-time code, so the domain still matters.

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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.