AQA A-Level Computer Science Paper 2, June 2025: Question 8

12 marks · Hard difficulty · Extended Response

Explain IPv4 address exhaustion, how IPv6 solves it, the difference between public and private IP addresses, and how NAT and DHCP mitigate the address shortage.

Practise this question

Question

Question 08 presents an extended response question worth 12 marks. The prompt states that IPv4 does not provide enough unique addresses for every Internet-connected device, which is being addressed long term by IPv6 and short term by public/private IP addresses, NAT, and DHCP. Students are instructed to explain: why IPv4 lacks enough unique addresses and how IPv6 overcomes this; the difference between a public (routable) and private (non-routable) IP address; how NAT works and how it helps; and how DHCP works and how it helps.
Question text

08 The Internet Protocol version 4 (IPv4) addressing system does not provide enough

unique addresses for every device that is connected to the Internet.

This is being addressed in the long term by the replacement of IPv4 with IPv6.

In the short term, the use of public and private IP addresses, NAT and DHCP is

helping to deal with this problem.

Explain:

• why IPv4 does not provide enough unique addresses and how IPv6

will overcome this problem

• the difference between a public (routable) and a private (non-routable) IP address

• how NAT works and how it helps to deal with this problem

• how DHCP works and how it helps to deal with this problem.

[12 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 08 showing a 4-level mark grid for 12 marks based on AO1 understanding: Level 4 (10–12 marks), Level 3 (7–9 marks), Level 2 (4–6 marks), and Level 1 (1–3 marks). Indicative content is divided into four areas: Area 1 covers IPv4 vs IPv6 bit lengths (32 bits vs 128 bits) and address space (2^32 vs 2^128); Area 2 covers public versus private IP addresses in terms of uniqueness and routability; Area 3 explains NAT operation using IP and port translation tables to share a single public IP; Area 4 explains DHCP's automatic allocation and leasing of IP addresses from a pool.

Total

Qu Pt Marking guidance

marks

08 Marks are AO1 (understanding) 12

Mark

Level Description

Range

4 A line of reasoning has been followed to produce a 10–12

coherent, relevant, substantiated and logically structured

response. The response covers all four areas indicated in

the guidance below and, in at least three areas, there is

sufficient detail to show that the student has a good level of

understanding.

3 A line of reasoning has been followed to produce a 7–9

coherent, relevant, substantiated and logically structured

response which shows a good level of understanding of at

least two areas indicated in the guidance below and some

understanding of at least one other area.

2 A limited attempt has been made to follow a line of 4–6

reasoning and the response has a mostly logical structure.

A good level of understanding has been shown of one area

and some understanding has been shown of a second area

OR some understanding of three areas.

1 A few relevant points have been made but there is no 1–3

evidence that a line of reasoning has been followed.

Guidance – Indicative Content

Area 1: Why IPv4 does not provide enough addresses and how IPv6

overcomes this

• IPv4 does not use enough bits to be able to represent a unique value for every

device

• IPv4 addresses use 32 bits / 4 bytes and IPv6 addresses use 128 bits / 16

bytes // IPv6 addresses uses 96 more / four times as many bits than IPv4

addresses // IPv4 (theoretically) allows 232 / 4 294 967 296 (unique) addresses

and IPv6 (theoretically) allows 2128 (unique) addresses.

A good understanding would be demonstrated by explaining that IPv6 addresses

are stored in more memory than IPv4 addresses and quantifying this correctly for

at least one of IPv4 or IPv6 in terms of either the number of bits/bytes used or the

number of available addresses.

Area 2: The difference between a public and private IP address

• Public IP address globally unique.

• Private IP address only locally unique // only unique within LAN / private

network.

– A-LEVEL COMPUTER SCIENCE – –

• Public IP addresses can be routed to / are routable from external networks / the

Internet

• Private IP addresses cannot be routed to / are not routable from external

networks / the Internet // private IP addresses can only be routed to within the

same network.

A good understanding would be demonstrated by describing both the scopes in

which the types of IP address must be unique and their routability.

Area 3: NAT

• NAT operates on routers that connect a LAN / network to the Internet.

• When a NAT router receives a request from a computer on the LAN/network to

access a resource on the Internet, it will replace the (private) IP address of the

computer / the (private) source IP address with the (public) IP address of the

router.

• The NAT router also replaces the (source) port number with a port number it

generates.

• The router adds the mapping (port number to private IP address and port

number / socket) it has created to its translation table.

• When a reply is received, it is recognised by its destination port number // the

(destination) port number is looked up in the translation table.

• (If the port number is present in the translation table) the reply is forwarded to

the computer that made the original request / the (public) destination IP address

is replaced with the (private) IP address of the computer that made the original

request (and the destination port number is replaced with the one stored in the

NAT translation table / the original source port number).

• How helps: Replacing the (private) IP address when data is sent from a

computer on the LAN onto the Internet means that the computers on the LAN

do not need to have globally unique IP addresses // only the (external) router

needs to have a globally unique IP address // allows computers with private IP

addresses to be connected to / send data to / receive data from (other

computers on) the Internet.

A good understanding would be demonstrated by describing how NAT works

mostly correctly and stating how NAT helps address the problem of a limited

number of IPv4 addresses.

Area 4: DHCP

• DHCP allows the automatic configuration of (the network settings / IP

addresses) of computers on a LAN.

• Computer (joining the network) sends request to discover a (DHCP) server.

• DHCP server maintains pool of IP addresses.

• DHCP server(s) offer configuration (A. IP address) to host.

• Host accepts offer of configuration (A. IP address) from (a DHCP) server (by

echoing the accepted configuration back to the server).

• (DHCP) server confirms that configuration (A. IP address) has been allocated

to host.

• How helps: DHCP helps deal with the problem of there not being enough IPv4

addresses by facilitating the reuse of IP addresses when a computer leaves a

network // DHCP only leases an IP address to a computer for a fixed time–A-LEVELCOMPUTER SCIENCE– –

period.

A good understanding would be demonstrated by describing how DHCP works

mostly correctly (there might be some omissions, for example in relation to the 25

confirmation of the configuration at the end of the process) and stating how DHCP

helps address the problem of a limited number of IPv4 addresses.

How to answer it

Addressing IPv4 Exhaustion: IPv6, Public/Private IPs, NAT & DHCP

Question 08 • 12 Marks • AO1 (Understanding) • Level of Response

What this question tests

This 12-mark extended response evaluates comprehensive knowledge of network addressing challenges and solutions. Specifically, you must understand:

  • The mathematical and structural causes of IPv4 address exhaustion and how IPv6 solves this long-term.
  • The precise distinctions in scope (uniqueness) and routability between public and private IP addresses.
  • The mechanics of Network Address Translation (NAT) using IP translation and port mappings.
  • The protocol operation of Dynamic Host Configuration Protocol (DHCP) via dynamic IP leasing.

Examiner Level Criteria (12-Mark Scheme)

How the 4 levels are determined for this question

🧠 Level 4 (10–12 marks)

Covers all 4 areas with a logical, coherent line of reasoning. Requires good detail in at least 3 areas.

🧠 Level 3 (7–9 marks)

Shows a good level of understanding in at least 2 areas, and some understanding of at least one other area.

🧠 Level 2 (4–6 marks)

Good understanding of 1 area, OR some understanding of 2–3 areas. Partially structured.

❌ Level 1 (1–3 marks)

Isolated facts or points made without a coherent line of reasoning. Omits multiple major areas.

Area 1: IPv4 Address Exhaustion vs. IPv6

Why IPv4 does not provide enough addresses and how IPv6 overcomes this

💡 Key Knowledge

  • IPv4 architecture: Uses 32 bits (4 bytes), yielding 2³² addresses (~4.29 × 10⁹ or 4,294,967,296 unique addresses).
  • The problem: The explosion of internet-connected devices (smartphones, IoT, computers) far exceeds 4.3 billion. IPv4 does not have enough bits to provide a unique address for every connected device globally.
  • IPv6 solution: Uses 128 bits (16 bytes), providing 2¹²⁸ addresses (~3.4 × 10³⁸).
  • IPv6 has 4 times as many bits (96 more bits) than IPv4, providing an practically inexhaustible pool of unique public addresses.

📐 Mathematical Comparison

IPv4: 32 bits = 2³² ≈ 4.29 × 10⁹ addresses
IPv6: 128 bits = 2¹²⁸ ≈ 3.40 × 10³⁸ addresses

Trap: Saying IPv6 has "4 times more addresses" is false. It has 4 times more bits, which means 2⁹⁶ times more addresses!

❌ Common Misconceptions

  • Failing to state the exact bit-lengths (32 bits vs 128 bits) or stating incorrect powers of 2.
  • Confusing address length with address space (e.g., writing "IPv6 provides four times as many IP addresses").

Area 2: Public vs Private IP Addresses

Routability and Uniqueness Scopes

✅ Public (Routable) IP Addresses

  • Scope of Uniqueness: Must be globally unique across the entire Internet. No two devices online can share the same public IP.
  • Routability: Directly routable across external networks and the global Internet backbone.
  • Assigned by network providers/ISPs (managed by ICANN/IANA).

✅ Private (Non-Routable) IP Addresses

  • Scope of Uniqueness: Only locally unique within a single Local Area Network (LAN) / private network.
  • Routability: Cannot be routed on the public Internet; traffic is discarded by Internet routers.
  • Conservation impact: Different private networks can reuse the exact same private address space (e.g. 192.168.x.x , 10.x.x.x ) without collision.
Examiner Insight to hit "Good Understanding": You must explicitly contrast both criteria: (1) the scope of uniqueness (globally unique vs locally unique) AND (2) routability (routable on the Internet vs only routable inside the LAN).

Area 3: Network Address Translation (NAT)

How NAT operates step-by-step and alleviates address shortages

💡 Step-by-Step NAT Mechanism

Step 1 (Outbound Request): A host on a LAN (e.g., 192.168.1.15:52000 ) sends a packet to a web server across the Internet.
Step 2 (Translation & Mapping): The NAT router intercepts the packet. It replaces the private source IP with its own single public IP, and replaces the private source port with a newly allocated port.
Step 3 (NAT Table Entry): The router records this mapping into its NAT translation table:
Private IP:Port [192.168.1.15:52000] ⇄ NAT Public Port [82.10.x.x:61001]
Step 4 (Inbound Reply): When the remote server replies, it addresses the router's public IP and translated port. The NAT router looks up this port in the translation table, translates the destination IP/port back to the host's private IP/port, and forwards the packet onto the LAN.

✅ How NAT Helps Conservation

An entire network containing hundreds of devices requires only one single public IP address on the external router interface. This drastically reduces the demand for globally unique IPv4 addresses.

❌ Common Errors

  • Forgetting to mention the use of port numbers (sockets) to distinguish between multiple internal devices sharing the single public IP.
  • Describing NAT only as a security firewall rather than explaining the translation table mechanism.

Area 4: Dynamic Host Configuration Protocol (DHCP)

How DHCP works and conserves address space

💡 The 4-Step DHCP Handshake (DORA)

  1. Discover: Host joins the network and broadcasts a request to discover an active DHCP server.
  2. Offer: The DHCP server holds a pool of available IP addresses and offers one (with subnet mask, gateway, DNS) to the host.
  3. Request: The host echoes back an acceptance requesting the offered configuration.
  4. Acknowledge (Confirm): The server confirms the IP allocation has been leased to the host.

✅ How DHCP Helps Conservation

  • Addresses are leased temporarily rather than permanently allocated.
  • When a device disconnects or leaves the LAN, its IP address expires and returns to the available pool.
  • Enables address reuse: an organization does not need an IP address for every device ever connected, only enough for devices actively connected at any one time.
Examiner Insight: To score top band, explain both the mechanism (server manages pool, client discovers, lease granted/confirmed) and explicitly state how it conserves IPs (addresses are recycled/reused when devices disconnect).

🧠 Level 4 Synthesis Checklist

Ensure your 12-mark essay covers all four required pillars with explicit cause-and-effect language:

  • ✔ Area 1: Stated IPv4 bit size (32 bits / 2³²) vs IPv6 bit size (128 bits / 2¹²⁸) and explained the address shortage.
  • ✔ Area 2: Contrasted public (globally unique, routable externally) vs private (locally unique, non-routable on the Internet).
  • ✔ Area 3: Explained NAT router table mapping private IP/port to public IP/port, noting only the router requires a public IP.
  • ✔ Area 4: Described DHCP pool leasing mechanism and explained how address reclamation/reuse prevents waste.

Topics

4.9 Fundamentals of communication and networking · 4.9.4 The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol

Question and mark scheme from the AQA A-Level Computer Science examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.