← ACARA Digital Technologies Years 9–10

Subnet mask

A number like 255.255.255.0 that splits an IP address into a network part and a device (host) part. With 255.255.255.0, the first three groups name the network.

Part of: ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 25, WA Computer Science ATAR Year 11 — Network Communications — Module 48

Understanding Subnet mask in ACARA Digital Technologies Years 9–10

What it does: Like an IPv4 address, the mask has four groups separated by dots, so 255.255.255.0 is a correctly formatted subnet mask. Both addresses begin with 192.168.1, so under 255.255.255.0 they are on the same network and can join the same LAN party game. Only the device part (10 and 11) differs. The first three groups (192.168.1) are the network. The last group (25) is the device.

Common mistakes: A subnet mask is written like an IP address, with four groups separated by dots. It should be mask = 255.255.255.0. With the mask 255.255.255.0 only the last group is the device (host) part. The host is 20, not 192, which is part of the network.

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Learning
Duo
A subnet mask such as 255.255.255.0 is used to...
Hide an IP address from other devicesSplit an IP address into a network part and a device (host) part
Matches
Mask 255.255.255.0: network part of 192.168.1.25 is 25
↓
The network part is the first three groups (192.168.1), not the last group. The last group, 25, is the device (host) part.
Applying
Meanings
Term
subnet mask
Meaning
A number like 255.255.255.0 that splits an IP address into a network part and a device (host) part. With 255.255.255.0, the first three groups name the network.
Practice
LAN Party PC IP: 192.168.1.25
Testing
Gaps
Subnet mask:
Check Answer
Spots
1# LAN Party network
2mask = 255.255.255

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Quick Questions

1. A subnet mask such as 255.255.255.0 is used to...
A. Split an IP address into a network part and a device (host) part ✓
B. Hide an IP address from other devices
The mask tells us which part of the IP address names the network and which part names the device.
2. Which one is written like a subnet mask?
A. 255-255-255-0
B. 255.255.255.0 ✓
A subnet mask uses the same dotted format as an IPv4 address: four numbers separated by dots.
3. With the mask 255.255.255.0, which part of the address 192.168.1.25 is the network part?
A. 192.168.1 ✓
B. 25
The first three groups (192.168.1) are the network. The last group (25) is the device.
4. With the mask 255.255.255.0, which part of 192.168.1.25 is the device (host) part?
A. 192.168
B. 25 ✓
The last group identifies the device on the network. Here the device part is 25.
5. At a LAN party, your PC is 192.168.1.10 and your friend's PC is 192.168.1.20, both with mask 255.255.255.0. Are they on the same network?
A. Yes, both have the network part 192.168.1 ✓
B. No, because the last groups are different
Only the network part must match. The last group (10 and 20) just tells the two devices apart.

Match the Pairs

255.255.255.0 → A subnet mask: the first three groups of an IP address name the network and the last group names the device
192.168.1.25 with mask 255.255.255.0 → The network part is 192.168.1 and the device (host) part is 25
A subnet mask is written in the same dotted format as an IP address: four groups of numbers separated by dots → 255.255.255.0
At a LAN party, two laptops on the same network share the same network part, so their first three groups match under mask 255.255.255.0 → 192.168.1.10 and 192.168.1.11
Mask 255.255.255.0: network part of 192.168.1.25 is 25 → The network part is the first three groups (192.168.1), not the last group. The last group, 25, is the device (host) part.
Subnet mask: 255.255.255 → A subnet mask needs four groups like an IP address. This one is missing the last group and should be 255.255.255.0.

Key Term

subnet mask
A number like 255.255.255.0 that splits an IP address into a network part and a device (host) part. With 255.255.255.0, the first three groups name the network.

Fill in the Blanks

1. IP address: 192.168.1.20 Subnet mask: 255.255.255.0
Hint: Type the mask that marks the first three groups as the network part.
2. IP: 192.168.1.20 Mask: 255.255.255.0 Network part: 192.168.1
Hint: Type the first three groups of the IP address, which the mask says are the network.
3. IP: 192.168.1.20 Mask: 255.255.255.0 Host part: 20
Hint: Type the last group of the IP address, which names the device on the network.
4. PC1 IP: 192.168.1.10 PC2 IP: 192.168.1.11 Mask: 255.255.255.0 Same network part: 192.168.1
Hint: Type the word for the part of the address that both LAN Party PCs share.
5. IP address: 192.168.1.20 Mask: 255.255.255.0
Hint: Type the third group of the mask, which still counts as part of the network.

Spot the Error

Question 1
A # LAN Party network
B mask = 255.255.255
C ip = 192.168.1.20
Line B — A subnet mask is written like an IP address, with four groups separated by dots. It should be mask = 255.255.255.0.
Question 2
A ip = 192.168.1.20
B mask = 255.255.255.0
C network = 192.168.1
D host = 192
Line D — With the mask 255.255.255.0 only the last group is the device (host) part. The host is 20, not 192, which is part of the network.
Question 3
A ip = 192.168.5.7
B mask = 255.255.255.0
C network = 192.168
D host = 7
Line C — The mask 255.255.255.0 makes the first three groups the network part. The network is 192.168.5, not just 192.168.
Question 4
A mask = 255,255,255,0
B ip = 10.0.0.15
C network = 10.0.0
Line A — A subnet mask uses the same dotted format as an IP address, with dots between groups, not commas. It should be mask = 255.255.255.0.
Question 5
A # Same network check
B pcA = 192.168.2.10
C pcB = 192.168.1.25
D mask = 255.255.255.0
E same = true
Line B — With mask 255.255.255.0 the first three groups must match for the PCs to share a network. pcA has network 192.168.2 but pcB has 192.168.1, so same = true is wrong. pcA should be 192.168.1.10.

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