← ACARA Digital Technologies Years 9–10

Checking whether two devices are on the same network

Two devices are on the same network when their IP addresses have the same network part under the subnet mask. If the network part differs, data must go via the router.

Part of: ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 26, WA Computer Science ATAR Year 11 — Network Communications — Module 49

Understanding Checking whether two devices are on the same network in ACARA Digital Technologies Years 9–10

What it does: The network part 192.168.1 is shared, but the host parts 5 and 9 are different. Every device on the network has a unique host part. Devices on different networks cannot talk directly. Their data is sent to the default gateway (the router), which forwards it. Both addresses in the correct answer start with 192.168.2. In the other pair the third numbers (2 and 3) differ, so the networks differ.

Common mistakes: A subnet mask has four numbers, like an IPv4 address. It should be 255.255.255.0. With mask 255.255.255.0 the network parts are 192.168.1 and 192.168.2. They differ, so the line should say different networks.

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This module's key terms are also in a printable crossword (Modules 12–34) and word search (Modules 12–55).

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Learning
Duo
A subnet mask is 255.255.255.0. Are 192.168.1.5 and 192.168.1.9 on the same network?
No, the last numbers (5 and 9) are differentYes, both start with 192.168.1
Matches
192.168.1.9 and 192.168.2.9 are on the same network because both end in .9
↓
The host part (the last number) is not what decides the network. The network part is 192.168.1 versus 192.168.2, so they are on different networks.
Applying
Meanings
Term
same network
Meaning
Two devices are on the same network when their IP addresses have the same network part under the subnet mask. If the network part differs, data must go via the router.
Practice
Subnet mask: 255.255.255.0
Testing
Gaps
network part of both:
Check Answer
Spots
1mask = 255.255.255
2A = 192.168.1.5

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Quick Questions

1. A subnet mask is 255.255.255.0. Are 192.168.1.5 and 192.168.1.9 on the same network?
A. Yes, both start with 192.168.1 ✓
B. No, the last numbers (5 and 9) are different
With 255.255.255.0 the first three numbers are the network part. Both are 192.168.1, so they match. Only the last number (the host part) differs.
2. A subnet mask is 255.255.255.0. Are 192.168.1.5 and 192.168.2.9 on the same network?
A. Yes, both start with 192.168
B. No, the network parts 192.168.1 and 192.168.2 differ ✓
The third number is part of the network part with this mask. 1 and 2 are different, so the devices are on different networks.
3. Device A is on 192.168.1.x and Device B is on 192.168.2.x (mask 255.255.255.0). How does data get from A to B?
A. It must go via the router ✓
B. It goes straight across the switch
Devices on different networks cannot talk directly. Their data is sent to the default gateway (the router), which forwards it.
4. Which part of 192.168.1.5 is the host part when the mask is 255.255.255.0?
A. 5 ✓
B. 192.168.1
The mask covers the first three numbers (the network part). The last number, 5, identifies the device (the host) on that network.
5. Two devices on the same network can have the same host part, such as both using 192.168.1.5.
A. True: the network part makes them different
B. False: each host part must be unique in one network ✓
Within one network every device needs a unique host part. If two devices share 192.168.1.5 there is a duplicate address and data may go to the wrong device or fail.

Match the Pairs

192.168.1.5 and 192.168.1.9 with mask 255.255.255.0 → Same network: both start with 192.168.1, so they can talk directly
192.168.1.9 and 192.168.2.9 with mask 255.255.255.0 → Different networks: the network parts differ, so data must go via the router
Each device on one network needs its own unique host part → 192.168.1.5 and 192.168.1.9
Two devices with the same address on one network cause a duplicate address problem → 192.168.1.5 and 192.168.1.5
192.168.1.9 and 192.168.2.9 are on the same network because both end in .9 → The host part (the last number) is not what decides the network. The network part is 192.168.1 versus 192.168.2, so they are on different networks.
192.168.1.5 and 192.168.1.9 are on different networks because the last numbers differ → Different host parts do not mean different networks. The network part 192.168.1 is the same, so they are on the same network.

Key Term

same network
Two devices are on the same network when their IP addresses have the same network part under the subnet mask. If the network part differs, data must go via the router.

Fill in the Blanks

1. A: 192.168.1.5 B: 192.168.1.9 mask: 255.255.255.0 network part of both: 192.168.1
Hint: Type the first three numbers that both addresses share under this mask.
2. A: 192.168.1.5 B: 192.168.2.9 mask: 255.255.255.0 same network? no, so use the router
Hint: Type yes or no: do the network parts of these two addresses match?
3. A = 192.168.1.5, B = 192.168.1.9 Each host part must be unique in a network
Hint: Type the word meaning no other device on the network can share that host part.
4. A = 192.168.1.5, B = 192.168.1.5 Same address twice = duplicate address
Hint: Type the word for an address that is used by two devices at once.
5. if netA == netB: send directly else: send via the router
Hint: Type the device that passes data between different networks.

Spot the Error

Question 1
A mask = 255.255.255
B A = 192.168.1.5
C B = 192.168.1.9
D A and B: same network
Line A — A subnet mask has four numbers, like an IPv4 address. It should be 255.255.255.0.
Question 2
A mask = 255.255.255.0
B A = 192.168.1.5
C B = 192.168.2.9
D A and B: same network
E data goes via the router
Line D — With mask 255.255.255.0 the network parts are 192.168.1 and 192.168.2. They differ, so the line should say different networks.
Question 3
A mask = 255.255.255.0
B A = 192.168.1.5
C network part of A: 192.168.1.5
D host part of A: 5
Line C — The mask 255.255.255.0 makes the first three numbers the network part. That is 192.168.1. The last number, 5, is the host part.
Question 4
A PC1 = 192.168.1.5
B PC2 = 192.168.1.5
C Problem: different networks
D Fix: give PC2 192.168.1.9
Line C — Both PCs have exactly the same address, so they are on the same network. The problem is a duplicate address, not different networks.
Question 5
A mask = 255.255.255.0
B A = 192.168.1.5
C B = 192.168.1.9
D Host parts 5 and 9 must match
E A and B: same network
Line D — Devices on one network share the network part, but each needs a unique host part. The hosts 5 and 9 are different, which is correct.

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