← WA Computer Science ATAR Unit 1

Network devices and the layers they operate at

The OSI layer a device works at: hubs at layer 1, NICs and access points at layers 1-2, switches at 2, routers at 3, and firewalls and proxies at layers 3 to 7.

Part of: WA Computer Science ATAR Year 11 — Network Communications — Module 34

Understanding Network devices and the layers they operate at in WA Computer Science ATAR Unit 1

What it does: Layer 2 switches learn MAC addresses and use a MAC table. This is data link layer work. A layer 3 switch adds routing by IP address (network layer) to normal switching at layer 2. A hub is a layer 1 device. It does not read addresses at all, so it simply repeats the electrical signal to all other ports. Firewalls can filter by IP address (layer 3), port (layer 4) or even application content (layer 7), so they cover layers 3 to 7. A web proxy reads and forwards application data such as HTTP requests, so it sits at the Application layer (layer 7).

Common mistakes: A hub only repeats electrical signals, so it works at layer 1 (physical), not layer 2. It should be 'hub': 1. A wireless access point works at the physical and data link layers, which are layers 1 and 2. It should be [1, 2].

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This module's key terms are also in a printable crossword (Modules 315–470) and word search (Modules 100–470).

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Learning
Duo
A switch normally works at which layer of the OSI model?
Layer 2 (Data Link)Layer 3 (Network)
Matches
Hub: Layer 2 (reads MAC addresses)
↓
A hub does not read MAC addresses. It simply repeats signals at layer 1 (physical), so it is not a layer 2 device.
Applying
Meanings
Term
device layer
Meaning
The OSI layer a device works at: hubs at layer 1, NICs and access points at layers 1-2, switches at 2, routers at 3, and firewalls and proxies at layers 3 to 7.
Practice
Hub: Layer 1 (Physical)
Testing
Gaps
Hub: Layer (Physical)
Check Answer
Spots
1layers = {
2 'hub': 2,

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Quick Questions

1. A switch normally works at which layer of the OSI model?
A. Layer 2 (Data Link) ✓
B. Layer 3 (Network)
A standard switch forwards frames using MAC addresses, which are Data Link layer (layer 2) addresses.
2. Which statement about a hub is correct?
A. A hub works at the Physical layer and just repeats the signal out of every port ✓
B. A hub works at the Network layer and reads IP addresses to choose a port
A hub is a layer 1 device. It does not read addresses at all, so it simply repeats the electrical signal to all other ports.
3. Which device is mainly responsible for forwarding packets between networks using IP addresses?
A. Switch (Data Link layer)
B. Router (Network layer) ✓
Routers work at layer 3 (Network layer). They read IP addresses and decide the next hop between networks.
4. A layer 3 switch differs from a layer 2 switch because it can:
A. Only repeat signals to every port like a hub
B. Route traffic using IP addresses as well as switch using MAC addresses ✓
A layer 2 switch only uses MAC addresses. A layer 3 switch adds routing ability, so it can also make decisions based on IP addresses.
5. A network interface card (NIC) and a wireless access point operate at:
A. The Physical and Data Link layers (layers 1-2) ✓
B. The Transport and Application layers (layers 4-7)
NICs and access points send the signal (layer 1) and handle frames and MAC addresses (layer 2). They do not deal with ports or applications.

Match the Pairs

Hub: Layer 1 (Physical) → A hub only repeats electrical signals out of every port. It does not read addresses, so it works at the physical layer.
Router: Layer 3 (Network) → A router reads IP addresses in packets and chooses the next hop, so it works at the network layer.
A basic switch forwards frames using MAC addresses, so it works at the data link layer. → Switch (layer 2): MAC address table
A layer 3 switch can also route using IP addresses, so it works above the data link layer as well. → Layer 3 switch: forwards by IP address
Hub: Layer 2 (reads MAC addresses) → A hub does not read MAC addresses. It simply repeats signals at layer 1 (physical), so it is not a layer 2 device.
Firewall and proxy: Layer 1 only → Firewalls and proxies inspect addresses, ports and application data. They work at layers 3 to 7, not just the physical layer.

Key Term

device layer
The OSI layer a device works at: hubs at layer 1, NICs and access points at layers 1-2, switches at 2, routers at 3, and firewalls and proxies at layers 3 to 7.

Fill in the Blanks

1. Hub: Layer 1 (Physical)
Hint: Type the layer number where a hub works, because it only repeats electrical signals.
2. Layer 2 switch reads MAC addresses
Hint: Type the kind of address a layer 2 switch uses to forward frames.
3. Layer 3 switch and router use IP addresses
Hint: Type the kind of address used by devices working at the network layer.
4. Proxy and firewall: Layers 3 to 7
Hint: Type the highest layer number that proxies and firewalls can inspect.
5. Router works at the Network layer (3)
Hint: Type the name of the OSI layer where routers operate.

Spot the Error

Question 1
A layers = {
B 'hub': 2,
C 'switch': 2,
D 'router': 3
E }
Line B — A hub only repeats electrical signals, so it works at layer 1 (physical), not layer 2. It should be 'hub': 1.
Question 2
A device = 'access point'
B layers = [2, 3]
C print(device, layers)
Line B — A wireless access point works at the physical and data link layers, which are layers 1 and 2. It should be [1, 2].
Question 3
A sw_a = 'layer 2 switch'
B uses = 'IP address'
C layer = 2
Line B — A layer 2 switch forwards frames using MAC addresses. It does not read IP addresses, which is what a layer 3 device does. It should be 'MAC address'.
Question 4
A firewall = {
B 'name': 'packet filter',
C 'first': 3,
D 'last': 4
E }
Line D — Firewalls can inspect traffic from the network layer (3) up to the application layer (7). The last layer should be 7, not 4.
Question 5
A proxy = {
B 'type': 'application',
C 'layer': 2
D }
Line C — An application proxy works at the application layer, layer 7. It does not work at the data link layer (2). It should be 'layer': 7.

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