← ACARA Digital Technologies Years 9–10

Private and public IP addresses

An address like 192.168.1.5 that works only inside a local network. The internet sees the router's public IP instead, and NAT translates between the two so many devices can share it.

Part of: ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 24, WA Computer Science ATAR Year 11 — Network Communications — Module 47

Understanding Private and public IP addresses in ACARA Digital Technologies Years 9–10

What it does: NAT swaps the private address of a device for the router's single public address when data goes out to the internet. The private range is 172.16.0.0 to 172.31.255.255. The second number 20 is in that range, but 40 is outside it. Private addresses only work inside a local network, so different networks can reuse them without a clash. Public addresses must be unique. The whole 10.x.x.x block is reserved for private networks, just like 172.16-31.x.x and 192.168.x.x.

Common mistakes: Addresses starting 192.168 are in a private range, so the laptop's address is private. The line should be kind = "private". The private range runs from 172.16 to 172.31, so 172.60 is not private. It should be "172.16.0.0/12".

Class: _____________________  ·  Name: _____________________  ·  Date: _________

Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

🖨 Print worksheet + answer key Add Private and public IP addresses to a class →

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

Take this worksheet further — free with Cloodit

Each concept is reinforced from every angle — learned, then applied, then tested.

Learning
Duo
Which address is a private IP address?
192.168.1.58.8.8.8
Matches
Each phone and laptop at home has its own public IP address, so the internet sees 192.168.1.5
↓
192.168.1.5 is a private address, so the internet cannot see it. The internet sees only the router's public IP address.
Applying
Meanings
Term
private IP address
Meaning
An address like 192.168.1.5 that works only inside a local network. The internet sees the router's public IP instead, and NAT translates between the two so many devices can share it.
Practice
10.0.0.0 - 10.255.255.255
Testing
Gaps
192.168.1.5 is a IP address
Check Answer
Spots
1laptop = "192.168.1.5"
2kind = "public"

Cloodit turns curriculum-aligned coding content into journeys students work through one biome at a time.

The 'Peregrine Falcon' badge — Operator Ace, one of Cloodit's 106 collectible badges
Students earn collectable badges

Over 100 of them, across seven natural biomes.

Study Buddy
Students build a real study resource

Turns their work into revision material ready for Flash Cards and Exam study.

Diamond Dash — one of Cloodit's live, competitive Sprint games
Students lock in with Sprints

Live, competitive practice for the start of the lesson or a teacher-led lesson finisher.

A Cloodit certificate of achievement, ready to personalise and print
Custom printable certificates

Honour your top performers, or encourage a student who's trying hard — ready to print in seconds.

Add to class →

Quick Questions

1. Which address is a private IP address?
A. 8.8.8.8
B. 192.168.1.5 ✓
192.168.x.x is a private range used inside local networks. 8.8.8.8 is a public address that the internet can see.
2. Which statement about a public IP address is correct?
A. It is the address the internet sees for your network ✓
B. It only works inside your home network
A public IP address is how the wider internet sees your network. Private addresses only work inside the local network.
3. Which address falls inside a private range?
A. 172.20.4.9 ✓
B. 172.40.4.9
The private range is 172.16.0.0 to 172.31.255.255. The second number 20 is in that range, but 40 is outside it.
4. Ten devices in your house are online. How many public IP addresses does your home network usually need?
A. One, shared by all the devices ✓
B. Ten, one for each device
Each device has its own private address, but they all share the router's single public address.
5. What does NAT (network address translation) do?
A. Turns domain names into IP addresses
B. Translates between private and public addresses ✓
NAT is done by the router and swaps private addresses for the public one. Turning domain names into IP addresses is the job of DNS.

Match the Pairs

192.168.1.5 → A private IP address that works only inside a local network such as a home Wi-Fi network
10.0.0.12 → A private IP address from the 10.x.x.x range, often used in schools and large organisations
The router uses NAT so many devices on a home network can share one public IP address → NAT: 192.168.1.5 -> 203.0.113.8
The private range from 172.16 to 172.31 is reserved for local networks only → 172.20.4.9
Each phone and laptop at home has its own public IP address, so the internet sees 192.168.1.5 → 192.168.1.5 is a private address, so the internet cannot see it. The internet sees only the router's public IP address.
A web server on the internet is reached using the address 192.168.0.20 → 192.168.0.20 is a private address that only works inside a local network, so it cannot be used to reach a server on the internet.

Key Term

private IP address
An address like 192.168.1.5 that works only inside a local network. The internet sees the router's public IP instead, and NAT translates between the two so many devices can share it.

Fill in the Blanks

1. 192.168.1.5 is a private IP address
Hint: Type the word for addresses that only work inside a local network.
2. Private range: 10.0.0.0 to 10.255.255.255
Hint: Type the first number of the largest private range.
3. Private range: 172.16.0.0 to 172.31.255.255
Hint: Type the second number where this private range starts.
4. The router uses NAT to translate addresses
Hint: Type the three-letter abbreviation for network address translation.
5. Laptop: 192.168.1.5 (private) Router: 203.0.113.7 (public)
Hint: Type the word for the kind of address the internet sees.

Spot the Error

Question 1
A laptop = "192.168.1.5"
B kind = "public"
C print(kind)
Line B — Addresses starting 192.168 are in a private range, so the laptop's address is private. The line should be kind = "private".
Question 2
A lan_range = "10.0.0.0/8"
B home_range = "192.168.0.0/16"
C mid_range = "172.60.0.0/12"
Line C — The private range runs from 172.16 to 172.31, so 172.60 is not private. It should be "172.16.0.0/12".
Question 3
A router_public = "203.0.113.9"
B pc_private = "192.168.1.10"
C nat_job = "block all traffic"
Line C — NAT does not block traffic. It translates between private and public addresses, so it should be "translate addresses".
Question 4
A devices = 5
B public_ips_needed = devices
C # NAT shares one public IP
Line B — NAT lets many devices share one public address, so the home only needs 1 public IP, not one per device.
Question 5
A src = "192.168.1.5"
B dest = "93.184.216.34"
C # router applies NAT
D src = "192.168.1.5"
Line D — After NAT, the router swaps the private source address for its own public address. Line 4 should be src = "203.0.113.9".

Student with a class code? Join your class →