AQA A-Level Computer Science (7517)

## Learning Outcomes

- Explain the difference between **public IP addresses** and **private IP addresses**
- Identify why public addresses must be globally unique but private addresses only need local uniqueness
- Recognise the main **private IPv4 ranges** and avoid common exam mistakes about them
- Explain why a router or combined home device can have both a public and a private IP address
- Describe how **NAT** allows devices with private addresses to access the Internet
- Understand the role of **DHCP** in assigning IP addresses on a LAN
- Apply these ideas to exam-style scenarios involving home, school, and business networks

## AQA A-Level Computer Science (7517) Syllabus

For the AQA A-Level Computer Science (7517), you are expected to understand public and private IP addressing in IPv4 networks, with a focus on the following syllabus points:

- Know that an **IPv4 address** is a 32-bit address, usually written in dotted-decimal notation
- Explain the difference between **public/routable** and **private/non-routable** IP addresses
- Identify that public IP addresses must be globally unique, while private IP addresses only need to be unique within a local network
- Recognise the standard private IPv4 ranges: **10.0.0.0-10.255.255.255**, **172.16.0.0-172.31.255.255**, and **192.168.0.0-192.168.255.255**
- Understand why private addressing and **NAT** are widely used with IPv4
- Explain how a **router** translates between a private LAN and the public Internet
- Understand the role of **DHCP** in automatically assigning IP configuration to devices on a network
- Describe why a combined router device may have both a public and a private IP address

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between a public IP address and a private IP address?
2. Why can two computers on different home networks both use the address 192.168.1.10 without causing a problem?
3. Why does a home router often have both a public IP address and a private IP address?
4. How does NAT allow a laptop with a private IP address to receive a reply from a web server on the Internet?
5. Is the address 172.20.5.8 public or private, and why?

## Introduction

Every device that communicates using Internet Protocol needs an IP address. In AQA questions, the marks are usually not for giving a vague definition, but for showing that you understand how addressing works in a real network. That means being able to explain not just what a public or private address is, but why the distinction exists and how routers handle traffic between the two.

This topic matters because it links several Paper 2 ideas together: **IPv4 address shortage**, **DHCP**, **NAT**, routers, and local area networks. Students often know that 192.168 addresses are "private", but lose marks when asked to explain why they are non-routable or why a router needs two addresses.

> **Key Term: IP address**
> A numerical address used to identify a device or network interface on an IP network.

### IPv4, routability, and why public and private addresses both exist

For AQA, you should think mainly in terms of **IPv4**. An IPv4 address is **32 bits** long and is usually written in **dotted-decimal notation**, such as 192.168.1.24. Because 32 bits only allow about **4.3 billion** possible addresses, IPv4 addresses are limited. That shortage is one of the main reasons private addressing became so important.

> **Key Term: IPv4 address**
> A 32-bit IP address, usually written as four decimal numbers from 0 to 255 separated by dots.

A **public IP address** is an address that can be used on the Internet. Routers across the Internet can route packets to it because it is **globally unique**. If two different Internet-facing devices had the same public address, routers would not know where traffic should go. That is why public addresses must be unique worldwide.

A **private IP address** is used only inside a local network such as a home LAN, school network, or office network. It is **non-routable** on the public Internet. That means Internet routers do not forward packets addressed to these private ranges across the public Internet. A private address only has to be unique within its own LAN.

> **Key Term: public IP address**
> An IP address that is globally unique and routable on the Internet.
>
> **Key Term: private IP address**
> An IP address used within a local network that is not routed on the public Internet.
>
> **Key Term: routable**
> Able to be forwarded by routers across the Internet to reach another networked device.
>
> **Key Term: non-routable**
> Not forwarded across the public Internet; usable only within a local or restricted network.

The three standard private IPv4 ranges are:

- **10.0.0.0 to 10.255.255.255**
- **172.16.0.0 to 172.31.255.255**
- **192.168.0.0 to 192.168.255.255**

These are the only private IPv4 ranges you should use in an A-level answer. A common error is to say "any 172.x.x.x address is private" or "any 192.x.x.x address is private". That is wrong. Only the ranges above are private.

Why does this system help? Because the same private addresses can be reused in many separate networks. For example:

- Home A can have a printer at **192.168.1.50**
- Home B can also have a printer at **192.168.1.50**
- School C can have a laptop at **192.168.1.50**

This causes no conflict because those devices are on different local networks and their private addresses are not routed across the Internet. Uniqueness is only required inside each LAN.

That is a very exam-friendly comparison:

- **Public IP**: unique everywhere
- **Private IP**: unique only inside the local network
  > **Test Tip:** When a question says "explain the difference", aim to compare both **routability** and **uniqueness**. Many students only mention one and miss marks.

### Why a router can have both a public and a private IP address

One of the most common AQA questions is why a router, or a combined router/switch/hub device, has both a public and a private IP address. The short answer is that it connects **two different address spaces**.

On the **LAN side**, the router is part of the local network. It therefore uses a **private IP address**, such as 192.168.1.1, so devices inside the home or school can send data to it as their gateway.

On the **Internet side**, the router must communicate with the wider Internet. For this it needs a **public IP address**, usually assigned by the **ISP**. External servers and routers use this public address when sending replies back.

So the router sits between:

- a **private, internal network**
- a **public, Internet-facing network**

This is why it needs one address for each side.

A useful way to picture it is this:

- Laptop on LAN: **192.168.1.10**
- Phone on LAN: **192.168.1.11**
- Router LAN address: **192.168.1.1**
- Router WAN/public address: **81.24.115.9**

The laptop does not send traffic straight onto the Internet with 192.168.1.10 as a usable Internet return address. Instead, it sends the packet to the router, which then handles the public communication.

This setup also explains why most devices in a home do **not** need their own public IPv4 addresses. If every phone, console, smart TV, and printer needed a unique public IPv4 address, the shortage would be even worse.

> **Key Term: router**
> A networking device that forwards packets between networks and commonly connects a LAN to the Internet.
>
> **Key Term: ISP**
> Internet Service Provider; the organisation that supplies Internet access and often assigns a public IP address to a customer connection.

Students also confuse "the router has a private IP" with "the router is private". More accurately, the router usually has:

- a **private address on its internal interface**
- a **public address on its external interface**

That distinction matters because exam questions often ask about a "combined device" having two IP addresses.

### DHCP and automatic assignment of private addresses

In most LANs, devices are not configured manually with IP addresses. Instead, a **DHCP server** automatically assigns them. In a home network, the router often performs this role as well.

> **Key Term: DHCP**
> Dynamic Host Configuration Protocol; a system that automatically assigns IP configuration, such as an IP address and subnet mask, to devices on a network.

When a new device joins the network, it requests configuration. The DHCP server then offers an available address from its pool, along with other settings such as the subnet mask. Once accepted, the server records that assignment so it does not hand out the same address to another device at the same time.

This matters for private addressing because most internal devices use DHCP to obtain their **private IP addresses** automatically. For example, a laptop joining a home Wi-Fi network may receive:

- IP address: **192.168.1.24**
- subnet mask: **255.255.255.0**
- default gateway: **192.168.1.1**

Why is DHCP useful?

- It avoids manual setup on every device
- It reduces the risk of accidental duplicate addresses
- It allows addresses to be reused when devices disconnect
- It works well for mobile devices that connect and disconnect frequently

AQA sometimes asks why dynamic addresses are preferred over static addressing. The key idea is efficient use of limited address space and easier management. On a private LAN this means simpler administration. On public networks such as hotspots, it means addresses can be temporarily assigned and later returned to the pool.

Common student errors here include:

- saying DHCP only assigns public addresses
- forgetting that DHCP is also widely used on private networks
- mixing up DHCP with DNS

DHCP gives a device its network configuration. DNS translates domain names into IP addresses. These are different jobs.

> **Test Tip:** If an exam question asks how a device gets an address "when connecting for the first time", think **DHCP**, not NAT and not DNS.

### How NAT links private devices to the public Internet

A private IP address is non-routable, so a server on the Internet cannot directly send a reply to 192.168.1.10 as if that were a globally reachable address. This is why **Network Address Translation (NAT)** is needed.

> **Key Term: Network Address Translation (NAT)**
> The process of replacing one IP address with another as packets pass between a private network and the public Internet.

The basic logic of NAT is:

1. A device on the LAN sends a packet outward.
2. The packet has the device's **private source IP address**.
3. The router changes that source IP to its own **public IP address**.
4. The packet travels across the Internet using the router's public address.
5. The reply comes back to the router's public address.
6. The router translates the destination back to the original private device and forwards the packet internally.

Here is a worked example.

A laptop on a home network has private address **192.168.1.10**.  
The router's public address is **81.24.115.9**.

The laptop requests a web page from a server.

Before NAT:

- Source IP: **192.168.1.10**
- Destination IP: web server public address

After the packet reaches the router:

- Source IP becomes **81.24.115.9**
- Destination IP stays as the web server

The web server replies to **81.24.115.9**, because that is the source it saw. When the reply returns, the router checks its translation information and sends the data back to **192.168.1.10**.

This is the key reason private-addressed devices can still access Internet services.

In practice, routers also use **port numbers** to keep track of multiple simultaneous connections. If several devices are browsing at once, they may all share one public IP address, but the router can still tell which reply belongs to which device by using translation table entries involving addresses and ports.

You do not need to write a full essay on port number handling unless the question asks, but mentioning that the router tracks connections using a translation table is often strong exam technique.

Why is NAT so widely used?

- It allows many devices to share one public IPv4 address
- It reduces demand for scarce public IPv4 addresses
- It hides internal private addresses from the public Internet

Be careful with the last point. NAT can provide some protection because internal addresses are not directly exposed and unsolicited packets may be dropped if there is no matching translation entry. However, you should not overstate this and say NAT is the same thing as a firewall. In exam answers, safer wording is that NAT can provide **an additional layer of protection** or make internal devices less directly exposed.

A related idea is **port forwarding**. If a server exists inside a private network, outside users cannot directly reach its private IP address. The router can therefore be configured so that traffic arriving at a certain public port is forwarded to a chosen private IP and port inside the LAN. For example, requests arriving at the router on port 80 could be forwarded to an internal web server at **192.168.1.50**.

This matters because it shows the normal rule and the exception:

- Normally, private devices start the connection outward and NAT handles replies
- If an outside device needs to reach an internal server, the router must be configured to forward the traffic appropriately

## Key Point Checklist

_This article has covered the following key knowledge points:_

- An **IPv4 address** is a 32-bit address written in dotted-decimal notation
- A **public IP address** is globally unique and routable on the Internet
- A **private IP address** is used within a LAN and is non-routable on the public Internet
- Private IP addresses only need to be unique within their own local network
- The standard private IPv4 ranges are 10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255, and 192.168.0.0-192.168.255.255
- Not every 172.x.x.x or 192.x.x.x address is private; only the defined private ranges are
- A router often has a private IP address on its LAN side and a public IP address on its Internet side
- **DHCP** automatically assigns IP configuration such as an IP address and subnet mask
- **NAT** replaces a private source IP with the router's public IP for outgoing Internet traffic
- When replies return, the router translates the address back and forwards the data to the correct internal device
- NAT helps many devices share one public IPv4 address
- Private addressing and NAT are widely used because public IPv4 addresses are limited

## Key Terms and Concepts

- IP address
- IPv4 address
- public IP address
- private IP address
- routable
- non-routable
- router
- ISP
- DHCP
- Network Address Translation (NAT)


Communication and Networking - Public and private IP addresses