← WA Computer Science ATAR Unit 1

Designing a network from requirements

Turning user requirements into a plan by choosing devices, media, topology, IP addressing and security controls, then justifying each choice.

Part of: WA Computer Science ATAR Year 11 — Network Communications — Module 93

Understanding Designing a network from requirements in WA Computer Science ATAR Unit 1

What it does: A justified trade-off names the choice, the reason linked to a requirement, and what is given up. The device and its rule directly meet the requirement by blocking guests from reaching the staff network. Reliability is a requirement, so wired Ethernet is justified because it is less affected by interference than Wi-Fi. A design starts with a requirements list. Every later choice (devices, media, topology) is made to meet it. A switch connects many wired devices on a LAN. A modem connects the network to the ISP and is not the centre of the star. 192.168.x.x is a private range. It is used inside a LAN and is not routed on the public internet.

Common mistakes: A switch with only 8 ports cannot connect 20 users. The device must meet the requirement, so it should be a switch with 24 ports. Guests on a separate network should be segregated from internal resources. Their access should be internet only, not full LAN access.

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Learning
Duo
A school library needs 30 reliable computers with no dropped connections. Which cabling choice best meets this requirement?
Wi-Fi only, to save on cablingWired Ethernet to each computer
Matches
Design: 5 devices on 192.168.1.0/24. Gateway 192.168.1.256
↓
The default gateway 192.168.1.256 is invalid because each IPv4 octet must be 0 to 255; use a valid address such as 192.168.1.1.
Applying
Meanings
Term
network design
Meaning
Turning user requirements into a plan by choosing devices, media, topology, IP addressing and security controls, then justifying each choice.
Practice
Requirement: 20 staff, fast and secure
Testing
Gaps
topology:
Check Answer
Spots
1Requirement: 20 users
2Device: switch with 8 ports

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Quick Questions

1. A school library needs 30 reliable computers with no dropped connections. Which cabling choice best meets this requirement?
A. Wired Ethernet to each computer ✓
B. Wi-Fi only, to save on cabling
Reliability is a requirement, so wired Ethernet is justified because it is less affected by interference than Wi-Fi.
2. Which step comes FIRST when designing a network from user requirements?
A. List what the users need (devices, speed, security, budget) ✓
B. Buy the most expensive router available
A design starts with a requirements list. Every later choice (devices, media, topology) is made to meet it.
3. A small office needs 20 wired computers connected in a star topology. Which device should sit at the centre?
A. A network switch ✓
B. A modem
A switch connects many wired devices on a LAN. A modem connects the network to the ISP and is not the centre of the star.
4. A designer plans the IP address range 192.168.1.0 for an office LAN. Which type of address is this?
A. A public IPv4 address range
B. A private IPv4 address range ✓
192.168.x.x is a private range. It is used inside a LAN and is not routed on the public internet.
5. Which part of an IP address plan is the best way to give servers and printers a fixed address?
A. Let DHCP choose a new address each time
B. Assign static IP addresses ✓
Devices that others must always find, such as servers and printers, need static addresses. Ordinary laptops can use DHCP.

Match the Pairs

Requirement: 30 users, 100 Mbps each → A requirements list item that sets a measurable target, used to choose devices, cabling and bandwidth
192.168.10.0/24 - Staff, 192.168.20.0/24 - Guests → An IP address plan giving each group its own subnet so they can be kept separate
Justifying a trade-off: choose a wired star network for the office because it is faster and more reliable, even though cabling costs more → Choice: Ethernet star with switch | Reason: speed and reliability | Cost: higher cabling
Selecting a device to meet a security requirement: guest traffic must be kept away from the staff file server → Add router + firewall rule: DENY Guest subnet -> Staff subnet
Design: 5 devices on 192.168.1.0/24. Gateway 192.168.1.256 → The default gateway 192.168.1.256 is invalid because each IPv4 octet must be 0 to 255; use a valid address such as 192.168.1.1.
Requirement: staff data must be private. Design: free public Wi-Fi, no password → The design does not meet the security requirement; it needs WPA3 or WPA2 encryption, a strong password and separation from guests.

Key Term

network design
Turning user requirements into a plan by choosing devices, media, topology, IP addressing and security controls, then justifying each choice.

Fill in the Blanks

1. need: one cable fault must not stop others topology: star
Hint: Type the topology name where every device connects to a central switch.
2. network: 192.168.1.0/24 mask: 255.255.255.0
Hint: Type the subnet mask that matches the /24 prefix in the IP plan.
3. need: block outside traffic to the network control: firewall
Hint: Type the security control that filters traffic by rules.
4. need: link two buildings 400 m apart media: fibre optic cable
Hint: Type the cable type that carries light and suits long distances.
5. need: 100 laptops join and leave daily addressing: DHCP
Hint: Type the protocol that hands out IP addresses automatically.

Spot the Error

Question 1
A Requirement: 20 users
B Device: switch with 8 ports
C Medium: Ethernet cable
D Topology: star
Line B — A switch with only 8 ports cannot connect 20 users. The device must meet the requirement, so it should be a switch with 24 ports.
Question 2
A Need: guests use Wi-Fi
B Device: wireless access point
C Guest network: separate VLAN
D Guest access: full LAN access
Line D — Guests on a separate network should be segregated from internal resources. Their access should be internet only, not full LAN access.
Question 3
A Subnet mask: 255.255.256.0
B Network: 192.168.1.0
C Router: 192.168.1.1
D Printer: 192.168.1.50
Line A — Each octet of a subnet mask must be between 0 and 255, so 256 is invalid. The mask should be 255.255.255.0.
Question 4
A Subnet: /24 gives 24 hosts
B Devices needed: 120
C Result: subnet is big enough
Line A — A /24 subnet gives 254 usable host addresses, not 24. Only 254 hosts makes the result (big enough for 120 devices) correct.
Question 5
A Requirement: very fast links
B Medium: fibre optic cable
C Reason: high bandwidth
D Trade-off: lower cost
Line D — Fibre optic cable and its equipment cost more than copper Ethernet. The trade-off for the high bandwidth is higher cost.

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