Networking — A-Level Computer Science Study Guide

For: A-Level Computer Science candidates sitting A-Level Computer Science.

Covers: LAN vs WAN and network topologies, TCP/IP stack layers and roles, DNS and IP addressing, core application protocols (HTTP, FTP, SMTP), and network security mechanisms including encryption and firewalls.

You should already know: Basic programming concepts; one of Python / Java / VB.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the A-Level Computer Science style for educational use. They are not reproductions of past Cambridge International examination papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official Cambridge mark schemes for grading conventions.

1. What Is Networking?

A computer network is a set of interconnected computing devices (called nodes, including laptops, servers, routers, and IoT devices) that can exchange data and share resources according to standardized rules called protocols. Common synonyms include data communications network and computer network system. This topic is listed as Section 6 of the A-Level Computer Science syllabus, accounting for ~15% of marks on Paper 2 (written theory) and ~10% of marks on Paper 4 (practical theory). Questions range from 1-mark short answer definitions to 4-5 mark scenario-based analysis of real-world network setups.

2. LAN vs WAN; topologies

The two most common categories of networks tested on the A-Level Computer Science exam are Local Area Networks (LANs) and Wide Area Networks (WANs):

• LAN: Covers a small, contained geographic area (e.g. single home, office, school campus), owned and managed by a single organization, offers high transfer speeds (1Gbps to 100Gbps), low latency, and low error rates.
• WAN: Spans large geographic areas (e.g. across cities, countries, or globally), operated by third-party telecommunications providers, offers lower transfer speeds (1Mbps to 10Gbps for most commercial connections), higher latency, and higher error rates than LANs.

Network topology describes the structure of a network, split into physical topology (the actual layout of cables and hardware) and logical topology (the path data follows across the network). Four core topologies are tested:

1. Bus: All nodes connected to a single shared backbone cable. Pros: Low cost for small networks, easy to install. Cons: Backbone cable failure takes the entire network down, performance degrades as more nodes are added.
2. Star: All nodes connected to a central switch or hub. Pros: Easy to add/remove nodes without disrupting the network, single node failure does not affect other devices. Cons: Central switch is a single point of failure, requires more cabling than bus topologies.
3. Mesh: Every node connects directly to every other node (full mesh) or to multiple peer nodes (partial mesh). Pros: Maximum redundancy, no single point of failure, consistent high performance. Cons: Very high cost and complex to manage, reserved for critical infrastructure like internet backbone networks.
4. Ring: Nodes connected in a closed loop, data travels in one fixed direction. Pros: Predictable performance, no data packet collisions. Cons: Single break in the ring takes the entire network down, difficult to troubleshoot.

Worked Example

A 15-person café wants to set up a network for its point-of-sale systems and free customer Wi-Fi, with a limited budget and no dedicated IT staff. Which topology is most appropriate? Solution: Star topology. Justification: It is low-cost for small networks, easy to troubleshoot if a single device loses connection, and new customer devices can connect to the central wireless access point without disrupting other users.

3. TCP/IP stack — layers and roles

The TCP/IP stack is the global standard protocol suite that powers the internet and all modern computer networks. A-Level explicitly tests the 4-layer TCP/IP model (not the 7-layer OSI model unless specified) for all A-Level Computer Science networking questions, ordered from top (closest to the user) to bottom (closest to physical hardware):

1. Application Layer: Interacts directly with user-facing software, provides network services to applications. Core protocols include HTTP, FTP, SMTP, and DNS. Its role is to format data so the receiving application can interpret it, and handle user requests like loading a webpage or sending an email.
2. Transport Layer: Manages end-to-end communication between two devices, ensuring reliable or low-latency data transfer as needed. It uses two core protocols:

• TCP (Transmission Control Protocol): Connection-oriented, uses a 3-way handshake to establish a link before data transfer, retransmits lost packets and reorders out-of-sequence packets for full reliability. Used for web traffic, email, and file transfers.
• UDP (User Datagram Protocol): Connectionless, no error correction or retransmission, offers very low latency. Used for video calls, live streaming, and online gaming. Port numbers (16-bit identifiers) are used at this layer to route incoming data to the correct application on a device (e.g. port 443 for HTTPS traffic to a web browser).

3. Internet Layer: Handles addressing and routing of data packets across multiple networks. The core protocol is IP (Internet Protocol), which assigns unique IP addresses to every device, and uses routers to forward packets along the shortest available path to their destination.
4. Link Layer (Network Access Layer): Manages physical transmission of data between adjacent devices on the same LAN. It includes hardware like Ethernet cables and Wi-Fi adapters, and protocols like Ethernet and Wi-Fi 6, which convert IP packets into physical frames for transmission, using MAC addresses to identify adjacent devices.

Worked Example

Trace the flow of a request to load a webpage through the TCP/IP stack on the user's device:

1. Application layer: The user's web browser generates an HTTP GET request for the webpage, and passes the request data to the transport layer.
2. Transport layer: TCP wraps the request in a segment, adds a random source port (e.g. 49872) and destination port 443 (for HTTPS), adds a sequence number for error correction, and passes the segment to the internet layer.
3. Internet layer: IP wraps the segment in a packet, adds the user's private source IP (e.g. 192.168.1.12) and the public destination IP of the web server (e.g. 203.0.113.78), and passes the packet to the link layer.
4. Link layer: The Wi-Fi adapter wraps the packet in a frame, adds the source MAC address of the user's Wi-Fi card and destination MAC address of their home router, and transmits the frame over Wi-Fi to the router.

4. DNS and IP addressing

Every device connected to a network has a unique IP address used to identify it for data transfer. The A-Level Computer Science syllabus covers two versions of IP:

• IPv4: 32-bit address, written as 4 decimal octets separated by dots (e.g. 192.168.1.1), with a total of ~4.3 billion unique addresses, which are now fully exhausted globally.
• IPv6: 128-bit address, written as 8 groups of 4 hexadecimal digits separated by colons (e.g. 2001:db8::1), with a total of $3.4\times 10^{38}$ unique addresses, designed to replace IPv4.

IP addresses are split into public and private: Public IPs are assigned by your ISP, are unique globally, and are used for communication over the internet. Private IPs are used only within a LAN, are not routable over the public internet, and are reserved in the ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.

The Domain Name System (DNS) is a global distributed system that translates human-readable domain names (e.g. owlsprep.com) into machine-readable IP addresses, so users do not need to memorize long IP addresses to access websites. The DNS query process follows these steps:

1. The user's device checks its local DNS cache to see if it already has a stored IP for the requested domain.
2. If no record is found, the device sends a query to a recursive DNS server (usually operated by the user's ISP).
3. If the recursive server does not have the record cached, it queries a root DNS server, which directs it to the top-level domain (TLD) server for the domain's suffix (e.g. the .com TLD server for owlsprep.com).
4. The TLD server directs the recursive server to the authoritative DNS server for the specific domain, which returns the correct IP address.
5. The recursive server sends the IP back to the user's device, caches the record for future requests, and the user's browser connects to the web server IP.

Worked Example

A user reports they can access Google by typing 142.250.185.14 directly into their browser, but get an error when typing google.com. What is the most likely cause of the issue? Solution: DNS server failure. Justification: The user can connect to the Google server directly via IP, so their internet connection is working, but the translation of the domain name to an IP address (the core function of DNS) is failing.

5. Protocols — HTTP, FTP, SMTP

A protocol is an agreed set of rules that govern how data is transmitted between devices, ensuring devices from different manufacturers can communicate seamlessly. Three core application layer protocols are tested in the A-Level Computer Science syllabus:

1. HTTP (Hypertext Transfer Protocol): Used to transmit web content (HTML, images, CSS, JavaScript) between web servers and web browsers. It uses port 80 by default, and is unencrypted. HTTPS (HTTP Secure) is the encrypted modern standard, uses TLS/SSL encryption, uses port 443, and is required for all websites handling sensitive user data. Common HTTP methods include GET (to retrieve data from a server) and POST (to submit data to a server).
2. FTP (File Transfer Protocol): Used to transfer large files between devices over a network. It uses port 21 for control commands and port 20 for data transfer by default, and is unencrypted. SFTP (SSH File Transfer Protocol) is the secure alternative, uses SSH encryption, runs on port 22, and is used for all sensitive file transfers (e.g. uploading website files to a server).
3. SMTP (Simple Mail Transfer Protocol): Used only to send email from a client device to an email server, and to forward email between servers. It uses port 25 for unencrypted traffic and port 587 for TLS encrypted traffic. Note that SMTP is not used to receive email: POP3 (port 110/995) or IMAP (port 143/993) are used for retrieving email to a client device, a distinction examiners test frequently.

Worked Example

A photographer needs to send a 3GB raw photo portfolio to a client, then send a short confirmation email to the client. Which protocols should they use for each task, and why? Solution: Use SFTP for the portfolio transfer, SMTP for the confirmation email. Justification: SFTP is optimized for large file transfers, supports resuming interrupted transfers, and encrypts data in transit. SMTP is designed specifically for sending short email messages, and has attachment size limits that would block a 3GB file.

6. Network security — encryption, firewalls

Network security refers to measures taken to protect networks, devices, and data from unauthorized access, modification, or theft. Two core mechanisms tested in the A-Level Computer Science syllabus are encryption and firewalls:

1. Encryption: The process of converting readable plaintext data into unscrambled ciphertext using a mathematical algorithm and an encryption key, so only authorized parties with the correct decryption key can convert it back to readable plaintext. There are two types of encryption:

• Symmetric encryption: Uses the same secret key for both encryption and decryption. It is very fast, making it ideal for encrypting large volumes of bulk data. The most common symmetric standard is AES (Advanced Encryption Standard).
• Asymmetric encryption: Uses a pair of mathematically linked keys: a public key that can be shared openly (used to encrypt data) and a private key that is kept secret by the owner (used to decrypt data). It eliminates the need to share secret keys securely, but is much slower than symmetric encryption. The most common asymmetric standard is RSA. Real-world systems like HTTPS use a mix of both: asymmetric encryption is used to exchange a temporary symmetric session key between two devices, then symmetric encryption is used for all subsequent data transfer for speed.

2. Firewalls: A security tool (either hardware or software) that monitors all incoming and outgoing network traffic, and allows or blocks traffic based on predefined security rules. There are two types:

• Network firewalls: Hardware devices placed between a private LAN and the public internet, filter traffic for all devices on the network.
• Host-based firewalls: Software installed on individual devices (laptops, servers), filter traffic only for that specific device. Firewall rules can be based on IP addresses, port numbers, protocols, or application types. For example, a firewall rule can block all incoming traffic to port 22 (used for SSH remote access) from public IP addresses, only allowing access from the office LAN IP range.

Worked Example

A bank wants to protect its internal customer database server from unauthorized access over the internet. Name two security measures it should use, and explain how each works. Solution: 1. Encryption: All customer data stored on the server should be encrypted at rest using AES, so even if an attacker gains access to the physical server, they cannot read the data without the encryption key. 2. Network firewall: The bank can configure a firewall rule that blocks all incoming traffic to the database server's ports from public IP addresses, only allowing access from pre-approved internal bank IP addresses.

7. Common Pitfalls (and how to avoid them)

• Pitfall: Listing layers from the 7-layer OSI model when asked about the TCP/IP stack. Why students do it: Many learning resources cover both models, leading to confusion. Correct move: A-Level Computer Science explicitly tests the 4-layer TCP/IP model (Application, Transport, Internet, Link) unless the question mentions OSI; memorize these four layers in order for all TCP/IP questions to avoid losing marks.
• Pitfall: Stating SMTP is used for both sending and receiving email. Why students do it: They assume a single protocol handles all email functions. Correct move: Use the mnemonic "SMTP = Send, POP/IMAP = Receive"; this distinction is tested in at least 1 question per exam paper.
• Pitfall: Mixing up symmetric and asymmetric encryption properties, saying asymmetric encryption uses the same key for encryption and decryption. Why students do it: They confuse the two encryption types when cramming. Correct move: Memorize "Symmetric = Same key, Asymmetric = A pair of keys", and remember asymmetric encryption is slower and used for key exchange, while symmetric is faster for bulk data.
• Pitfall: Describing DNS as a single central server. Why students do it: They do not understand the distributed DNS hierarchy. Correct move: Always explain DNS as a global network of root, TLD, and authoritative servers, not one single server, to get full marks in scenario-based questions.
• Pitfall: Confusing physical and logical topologies, assuming a star physical topology always uses star logical data flow. Why students do it: They assume the layout of cables matches the path data takes. Correct move: Explicitly distinguish physical (cable/hardware layout) and logical (data flow path) topologies in all answers; for example, a physical star network can use a logical ring flow if the switch forwards data in a circular order.

8. Practice Questions (A-Level Computer Science Style)

Question 1 (3 marks)

A secondary school operates a LAN for its 100 classroom computers and admin devices, and connects to the internet via a WAN link to a local ISP. (a) State two differences between a LAN and a WAN. (2 marks) (b) The school uses a star physical topology for its LAN. State one disadvantage of this topology for the school. (1 mark)

Solution

(a) Any two valid differences, 1 mark each:

• LAN covers a small geographic area (single school site) while WAN covers a large geographic area (connects school to ISP across the city)
• LAN is owned and managed by the school while WAN is operated by the ISP
• LAN has higher data transfer speeds than WAN
• LAN has lower latency than WAN (b) Any one valid disadvantage, 1 mark:
• The central switch is a single point of failure, so if it breaks the entire school network goes down
• Star topology requires more cabling than cheaper bus topology, increasing setup cost for the 100-device network

Question 2 (4 marks)

When a user types https://studentportal.school.edu into their browser, the device first sends a DNS query, then establishes a TCP connection to the web server. (a) Explain the role of the recursive DNS server in this process. (2 marks) (b) State two functions of the TCP 3-way handshake before data is transmitted. (2 marks)

Solution

(a) 1 mark for each point:

• The recursive DNS server queries the root, TLD, and authoritative DNS servers on behalf of the user's device to find the IP address associated with studentportal.school.edu
• It caches the IP address for a set period of time, so future requests for the same domain can be answered faster without querying the full DNS hierarchy (b) 1 mark for each valid function, max 2:
• Establishes a reliable, connection-oriented link between the user's device and the web server
• Verifies both devices are active and ready to send/receive data
• Synchronizes sequence numbers for both devices to enable error correction and ordered packet delivery

Question 3 (3 marks)

A small retail business allows remote staff to access the internal inventory server to update stock levels from home. (a) State one benefit of using HTTPS instead of unencrypted HTTP for remote access to the inventory server. (1 mark) (b) The business uses a host-based firewall on the inventory server. Explain how this can prevent unauthorized access to the server. (2 marks)

Solution

(a) 1 mark: HTTPS encrypts all data transferred between the remote staff device and the inventory server, so even if the data is intercepted in transit it cannot be read by unauthorized parties. (b) 2 marks for full explanation: The host-based firewall can be configured with rules that only allow incoming traffic to the server's web port from the pre-approved public IP addresses of remote staff, blocking all other incoming connection attempts from unknown IP addresses.

9. Quick Reference Cheatsheet

10. What's Next

The networking concepts covered in this guide are a foundational prerequisite for two later sections of the A-Level Computer Science syllabus: Section 7 (Internet of Things), which covers how connected IoT sensors and devices communicate over local and wide area networks to collect and transmit data, and Section 8 (Cybersecurity), which expands on the network security mechanisms you learned here to cover threat vectors, attack methods, and advanced mitigation strategies. A strong understanding of core protocols like HTTP is also required for practical programming tasks in Paper 4, where you may be asked to write code that sends API requests or interacts with network services.

If you struggle with any of the concepts in this guide, or want more personalized practice questions tailored to your weak areas, you can ask Ollie, our AI tutor, for customized explanations and example problems any time on the homepage. You can also explore our full library of A-Level Computer Science study guides covering topics like data structures, algorithms, operating systems, and object-oriented programming to build a complete knowledge base for your A-Level Computer Science exams.

Aligned with the Cambridge International AS & A Level Computer Science 9618 syllabus. OwlsAi is not affiliated with Cambridge Assessment International Education.