DSE ICT — Computer Networks and Network Security: Lesson Prep Guide (Tutor Edition)

2026-11-03

Tutor lesson-prep guides · Secondary ICT

Students know that logging in needs a password, yet cannot say how many hosts a /26 holds, how long 250 MB takes, or how AES differs from RSA. Four calculable numbers turn networking and security from slogans into arithmetic and judgement.

Objectives — Students will be able to: (1) compute usable hosts from CIDR notation (254 for /24, 62 for /26, 14 for /28, 2 for /30) and justify subtracting two; (2) derive a theoretical 20 seconds and a realistic 25 seconds for a 250 MB file over 100 Mbps, naming the losses; (3) distinguish symmetric encryption (one key, fast, bulk data) from asymmetric encryption (key pair, slow, key exchange) and explain a login flow using a 32-byte hash and a default-deny firewall.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Binary	How many bits in a byte?	Eight, giving 0 to 255
IP addressing	How long is an IPv4 address?	32 bits, four octets
Units	How do Mbps and MB per second differ?	A factor of eight; lower-case b is a bit
Security basics	Authentication versus authorisation?	Who you are versus what you may do

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0–10 min	Bit warm-up	Ask students to write 255 in binary	Write 11111111
10–22 min	Subnet arithmetic	Push /24 to /26, /28 and /30 on the board	254, 62, 14, 2
22–32 min	Transfer time	250 MB over 100 Mbps	20 s theoretical, 25 s realistic
32–44 min	Encryption and hashing	Same message, two schemes, compare key length and speed	State the trade-off
44–52 min	Question drill	One subnet, one timing, one design, one judgement item	Work independently for eight minutes
52–60 min	Exit check	Mark three written items on the spot	Complete independently

03 Teaching script (read it out)

Scene 1: Why two addresses disappear

A /24 holds 256 addresses; the first is the network address and the last is broadcast, leaving 254 hosts. Each extra mask bit halves the range: /26 gives 64 addresses and 62 hosts, /28 gives 16 and 14, /30 gives 4 and 2, enough for a point-to-point link. Students should build these from powers of two rather than memorising the table.

Ask students: why are two addresses unusable?

Scene 2: Lower-case b is a bit

250 MB is 2000 Mb, so dividing by 100 gives 20 seconds. Overheads for headers, acknowledgements and retransmission mean eighty per cent efficiency is generous, so 25 seconds is the better answer. Distance, contention and wireless interference all slow it further.

Ask students: why is the real time always longer than the theoretical one?

Scene 3: Encryption is a division of labour

TLS does not choose one scheme: it uses asymmetric encryption to exchange a temporary key, then symmetric encryption such as AES for the data, because asymmetric operations are tens to hundreds of times slower. Passwords are never stored in plain text, only as a hash; SHA-256 always outputs 32 bytes, which is why a forgotten password can only be reset. A firewall is a rule table that denies by default and opens only required ports, with two-step verification and phishing training completing the picture.

Ask students: why not encrypt all traffic with asymmetric keys?

04 Question types and worked steps

Type	Example	Steps	Answer
Subnet calculation	How many hosts in a /26	Host bits equal 32 minus prefix, take the power of two, subtract two	62 hosts
Transfer time	250 MB over 100 Mbps	Multiply by eight, divide by bandwidth, then state a realistic figure	20 s, about 25 s in practice
Security design	Design login and payment for an online shop	HTTPS, hashed passwords, two-step verification, firewall and logs	Layered defence so one failure is not fatal
Judgement	Handling an account suspension email	Check the domain, do not click, log in from the official site, report it	Do not reply, do not click, verify independently

Sanity check (teach this): Sanity check: usable hosts are always fewer than addresses; multiply by eight before dividing by bandwidth or the answer is eight times too small; storage uses bytes and bandwidth uses bits; hashing is one way while encryption is reversible; deny-by-default beats opening everything and blocking later.

05 Independent practice (3 questions)

Question	Answer	What to watch
How many usable hosts in a /26?	62	Check two to the sixth power minus two
250 MB over 100 Mbps: theoretical time?	20 seconds	Check the factor of eight is applied
How many bytes does SHA-256 output?	32 bytes	Check 256 is divided by eight

06 Common mistakes and what to say

Mistake	Student writes	Say this
Forgetting to subtract two	A /24 connects 256 machines	The network and broadcast addresses cannot be assigned, so it is 254.
Dividing MB by Mbps	250 divided by 100 equals 2.5 seconds	Convert first: 250 MB is 2000 Mb.
Encrypting everything asymmetrically	RSA is safest, so use RSA for all data	RSA is far slower; use it for key exchange and AES for the payload.
Opening all ports first	Open everything, then close what breaks	Default deny and open only what is needed; open-all equals no firewall.

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — Copy three key terms from the networking chapter with one definition each
Data representation (S4 chapter) — Redo practice question 2 to sharpen binary conversion
Databases and SQL (S5 chapter) — From the index, explain least privilege for a database account

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many usable hosts in a /28?	14	If 16: reteach the minus two
250 MB over 100 Mbps, theoretical time?	20 seconds	If 2.5 seconds: reteach bits and bytes
Why can a password only be reset, not sent back?	Only the hash is stored, so the original cannot be recovered	If the company is unwilling: reteach hashing

09 Tutor FAQ

Which year level is this for?

S5 networking and security; S4 can use the subnet and unit sections alone.

Is 60 minutes enough?

Yes; if binary is weak, spend ten minutes on powers of two and move the design item to homework.

Is there practice attached?

The homework links to site chapter pages with answers under independent practice.

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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.