📄 Paper 2 · 4.9 Communication & Networking
4.9.3a TCP/IP, IP Addressing & Packet Switching
AQA 7517 · A-Level Computer Science · ~17 min read

The TCP/IP Model

The TCP/IP model is the practical protocol suite used on the internet. It has 4 layers (simpler than OSI's 7):

TCP/IP LayerOSI EquivalentProtocols
ApplicationApplication + Presentation + SessionHTTP, HTTPS, FTP, SMTP, DNS
TransportTransportTCP, UDP
Internet (Network)NetworkIP, ICMP
Link (Network Access)Data Link + PhysicalEthernet, Wi-Fi, ARP

IP Addresses — IPv4 vs IPv6

FeatureIPv4IPv6
Length32-bit128-bit
Format4 decimal octets: 192.168.1.18 groups of 4 hex: 2001:0db8:85a3::0000:8a2e:0370:7334
Addresses~4.3 billion (2³²)~340 undecillion (2¹²⁸)
Why needed?Exhausted by internet growthSolves address exhaustion
NAT needed?Yes — NAT extends IPv4 lifeNo — enough addresses for every device

Subnetting: divides a network into smaller sub-networks. The subnet mask identifies which part of an IP address is the network ID and which is the host ID. E.g. 255.255.255.0 means the first three octets are the network, last octet = host.

Packet Switching

Data is split into packets for transmission. Each packet contains:

  • Header: source IP, destination IP, packet number, total packets, protocol
  • Payload: the actual data (typically 1500 bytes max)
  • Trailer: error detection (CRC)

How it works:

  1. Data is split into packets at the source
  2. Each packet is routed independently — may take different paths
  3. Routers use routing tables to forward packets to the next hop
  4. Packets are reassembled in the correct order at the destination (TCP handles this)

Advantages of packet switching: efficient use of network (no dedicated circuit), resilient (packets reroute if a link fails), different packets take optimal paths.

DNS — Domain Name System

DNS is a hierarchical distributed database that maps domain names to IP addresses:

  1. User types cszone.co.uk in browser
  2. Browser checks local cache; if not found, queries recursive resolver
  3. Resolver queries root server → gets address of .uk TLD server
  4. TLD server returns address of authoritative name server for cszone.co.uk
  5. Authoritative server returns the IP address → browser connects

NAT — Network Address Translation

NAT allows multiple devices on a private LAN to share a single public IP address. The router translates private IPs (e.g. 192.168.x.x) to its public IP and tracks connections using a port translation table. This helps extend the life of IPv4.

Exam tip: AQA 7517 may ask you to explain packet switching and its advantages over circuit switching. Key points: packets take independent routes, network is efficient (no wasted dedicated lines), resilient to failures. For IPv4 vs IPv6: know the address sizes (32-bit vs 128-bit) and why IPv6 was introduced (address exhaustion). DNS resolution process is often asked in step-by-step form.
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Worksheet — 4.9.3a TCP/IP & Packet Switching

8 questions · instantly marked · AQA 7517 standard

Q1State three pieces of information typically found in the header of a data packet.[3]
✅ Mark scheme
Mark scheme
Any three from: source IP address [1]; destination IP address [1]; packet sequence number [1]; total number of packets [1]; protocol being used [1]; Time to Live (TTL) [1].
Q2Explain two advantages of packet switching over circuit switching for data transmission.[4]
✅ Mark scheme
Mark scheme
Any two: Efficient bandwidth use — no dedicated circuit reserved; many users share the same links [1+1]; Fault tolerance — if a link fails, packets can be rerouted via different paths [1+1]; No set-up time — data starts transmitting immediately without establishing a dedicated connection [1+1].
Q3Explain two differences between IPv4 and IPv6.[4]
✅ Mark scheme
Mark scheme
IPv4: 32-bit address [1] — ~4.3 billion possible addresses [1]. IPv6: 128-bit address [1] — ~340 undecillion possible addresses, solving address exhaustion [1]. IPv4 uses dotted decimal notation (e.g. 192.168.1.1); IPv6 uses hexadecimal groups [1].
Q4Describe the role of a router in packet switching.[3]
✅ Mark scheme
Mark scheme
Router: reads the destination IP address in each packet's header [1]; consults its routing table to determine the best next hop [1]; forwards the packet toward its destination, potentially via multiple routers [1].
Q5Outline the steps a browser takes when resolving a domain name using DNS.[4]
✅ Mark scheme
Mark scheme
1) Browser checks local DNS cache [1]; 2) If not found, queries recursive resolver [1]; 3) Resolver queries root server → gets TLD server address [1]; 4) TLD server returns authoritative name server address [1]; 5) Authoritative server returns the IP address → browser connects. (4 marks from these steps)
Q6Explain what NAT (Network Address Translation) does and why it is used.[3]
✅ Mark scheme
Mark scheme
NAT: allows multiple devices on a private LAN to share a single public IP address [1]; the router translates private IP addresses (e.g. 192.168.x.x) to its public IP when sending data out [1]; this extends the usable life of IPv4 by reducing the number of public IP addresses needed [1].
Q7Explain what is meant by a subnet mask and how it is used.[3]
✅ Mark scheme
Mark scheme
Subnet mask: a 32-bit number that identifies which part of an IP address is the network address and which part is the host address [1]; devices use a bitwise AND of the IP address and subnet mask to determine the network ID [1]; example: subnet mask 255.255.255.0 means first three octets = network, last octet = host [1].
Q8A data file is split into 5 packets during transmission. Explain how TCP ensures the file is correctly reassembled at the destination.[4]
✅ Mark scheme
Mark scheme
Each packet has a sequence number in its header [1]; packets may arrive out of order as they take different routes [1]; TCP at the destination uses sequence numbers to reorder packets [1]; if a packet is missing (not acknowledged), TCP requests retransmission from the sender [1].
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Mini Test — TCP/IP & Packets

10 questions · 10 minutes

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