What you'll learn
A computer network is two or more devices connected so that they can share data and resources. In OCR GCSE Computer Science (J277, section 1.3) you need to know the types of network (LAN and WAN), the factors that affect network performance, the client-server and peer-to-peer models, the hardware needed to build a network, the internet and the role of DNS, hosting and the cloud, the star and mesh topologies, wired and wireless connections, encryption, IP and MAC addressing, standards, protocols such as TCP/IP, HTTP, HTTPS, FTP, POP, IMAP and SMTP, and the concept of layers. This guide covers all of these and how they fit together.
Exam questions frequently ask you to compare topologies or network models, to explain why a protocol is needed, or to describe what happens when you visit a website. Clear definitions and the use of examples earn the marks.
Key terms and definitions
LAN (local area network) — A network covering a small geographical area, such as a single site, where the infrastructure is usually owned by the organisation.
WAN (wide area network) — A network covering a large geographical area, often using infrastructure leased from telecommunications companies; the internet is the largest WAN.
Client-server network — A network where clients request services and resources from a central server.
Peer-to-peer network — A network where all devices are equal and share resources directly with each other.
Router — Forwards data packets between networks.
Switch — Connects devices on a LAN and sends data to the correct device using its MAC address.
WAP (wireless access point) — Allows wireless devices to connect to a wired network.
NIC (network interface controller/card) — Hardware that allows a device to connect to a network.
DNS (Domain Name System) — Translates domain names into IP addresses.
Protocol — A set of rules that govern how data is transmitted across a network.
IP address — A logical address assigned to a device on a network, used to route data.
MAC address — A unique physical address assigned to a NIC by its manufacturer.
Encryption — Scrambling data using a key so that it cannot be understood if intercepted.
Core concepts
LANs and WANs
A LAN covers a single site, such as a school, office or home. The organisation owns the cabling and equipment. A WAN connects LANs over long distances, between towns, cities or countries, usually using connections leased from telecommunications providers (such as phone lines, fibre and satellite links). The internet is a WAN — a global network of networks.
Factors affecting network performance
- Bandwidth — the amount of data that can be sent per second. More bandwidth allows more data to move at once.
- Number of users — more devices sharing the same connection means less bandwidth for each.
- Transmission media — fibre-optic cable is faster than copper, and wired connections are generally faster and more reliable than wireless.
- Interference and distance — wireless signals weaken with distance and through walls, and are affected by other devices.
- Errors — if data is corrupted, it must be resent, which slows the network.
Client-server and peer-to-peer
Client-server: a central server stores files, manages user accounts, controls security and runs backups. Clients send requests and the server responds.
- Advantages: central management of security, users, software and backups; files can be accessed from any client; suited to large organisations.
- Disadvantages: servers are expensive to buy and maintain; a network manager is usually needed; if the server fails, clients lose access to its services.
Peer-to-peer: every computer has equal status and shares its own files and resources.
- Advantages: cheap and easy to set up; no dedicated server or specialist needed; suits small homes and offices.
- Disadvantages: no central control, so security and backups must be managed on each machine; files may be duplicated; performance suffers if a peer is busy or switched off.
Network hardware
- NIC — every device needs one to connect, wired or wireless.
- Switch — connects devices within a LAN, using MAC addresses to send data only to its intended recipient.
- Router — forwards packets between networks, using IP addresses to decide the next hop; it connects a LAN to the internet.
- WAP — provides wireless access to a wired network.
- Transmission media — Ethernet cables (copper), fibre-optic cables, and radio waves for Wi-Fi.
The internet, DNS, hosting and the cloud
The internet is a worldwide collection of interconnected networks. When you type a URL, your computer asks a DNS server to translate the domain name into the IP address of the web server. If that DNS server does not know it, the request is passed to other DNS servers until the address is found. Your browser then sends a request to the web server at that IP address, which returns the web page.
Hosting means storing a website or service on a server that is connected to the internet so that others can access it. The cloud refers to storage, services and applications provided remotely over the internet. Cloud services can be accessed from anywhere, scale easily and are maintained by the provider, but depend on an internet connection, involve ongoing costs and mean trusting a third party with data.
Star and mesh topologies
Star topology: each device is connected to a central switch.
- Advantages: if one cable fails, only that device is affected; easy to add devices; good performance because data goes only to its destination.
- Disadvantages: if the central switch fails, the whole network fails; uses a lot of cable.
Mesh topology: devices are connected to many other devices. In a full mesh, every device connects to every other.
- Advantages: data has many possible routes, so if one connection fails it can be rerouted; very reliable; no single point of failure.
- Disadvantages: expensive, with many connections; complex to set up and manage. Wireless mesh networks reduce the cabling problem and are used in homes to extend Wi-Fi.
Wired and wireless connections
Ethernet is the standard for wired LANs. It is fast, reliable and secure but restricts movement and requires cabling.
Wi-Fi uses radio waves. It allows freedom of movement and is easy to set up but is generally slower, affected by interference and range, and must be encrypted to protect data.
Bluetooth connects devices over short distances, such as headphones and keyboards, using little power.
Encryption scrambles data using an algorithm and a key; only devices with the correct key can read it. It is essential on wireless networks, where anyone in range could intercept the signal.
IP and MAC addresses
An IP address identifies a device on a network so that packets can be routed to it. IPv4 addresses are 32 bits, written as four denary numbers separated by full stops (for example 192.168.0.12). Because IPv4 does not provide enough addresses for every device, IPv6 uses 128 bits, written in hexadecimal.
A MAC address is a unique 48-bit address assigned to each NIC by its manufacturer, usually written as six pairs of hexadecimal digits. Switches use MAC addresses to deliver data within a LAN; routers use IP addresses to send data between networks.
Standards and protocols
Standards are agreed rules for hardware and software, so that devices from different manufacturers can work together. Protocols are the rules for how data is formatted and transmitted.
- TCP/IP — TCP splits data into packets, numbers them, checks they arrive and requests resends; IP addresses and routes the packets.
- HTTP — used by browsers to request and receive web pages.
- HTTPS — HTTP with encryption, protecting data such as passwords and card details.
- FTP — transfers files between computers.
- POP — retrieves emails from a server and usually deletes them from the server.
- IMAP — retrieves emails while keeping them on the server, so they can be accessed from several devices.
- SMTP — sends emails from a client to a server and between mail servers.
Layers
Networking is divided into layers, each with its own job and protocols, with each layer passing data to the next. Layers make networking easier to design and maintain because each layer can be changed without affecting the others; they also allow hardware and software from different manufacturers to work together, because each only needs to follow the standards for its layer. You do not need to memorise the names of the layers for J277, but you should explain why layering is used.
Worked examples
Example 1: Choosing a topology
A hospital needs a network that keeps working even if a cable is damaged. Recommend star or mesh and justify. (3 marks)
Answer: Mesh (1). Devices are connected to several others, so data can take a different route if one connection fails (1). There is no single point of failure, so critical systems remain available (1).
Example 2: Explaining DNS
Describe how DNS is used when a user enters a web address. (3 marks)
Answer: The domain name is sent to a DNS server (1). The DNS server looks up the matching IP address (or asks other DNS servers if it does not have it) (1). The IP address is returned to the browser, which uses it to request the web page from the web server (1).
Example 3: POP or IMAP
A user reads emails on a phone, a laptop and a tablet. Explain why IMAP is more suitable than POP.
Answer: IMAP keeps emails on the server, so the same messages can be viewed and organised on all three devices. POP usually downloads emails and removes them from the server, so they would only be available on one device.
Example 4: Network performance
Explain two reasons why a school's network slows down at lunchtime.
Answer: More students are using the network at the same time, so the available bandwidth is shared between more users. More data is being transmitted, increasing the chance of collisions and errors that require data to be resent.
Common mistakes and how to avoid them
- Mixing up switches and routers. Switches work within a LAN using MAC addresses; routers connect networks using IP addresses.
- Saying the internet and the World Wide Web are the same. The web is a service (web pages) running on the internet.
- Confusing POP and IMAP. IMAP keeps messages on the server; POP downloads them.
- Saying mesh "never fails". It is very reliable but expensive and complex.
- Forgetting that encryption needs a key.
Exam technique for "Computer Networks, Connections and Protocols"
- Learn each protocol's purpose in one sentence — that is usually what is asked.
- For comparisons, give an advantage and a disadvantage of each option, then decide in context.
- In "describe what happens" questions, sequence the steps clearly (DNS lookup → request → response).
- Use the correct terms: packets, bandwidth, IP address, MAC address.
Quick revision summary
- LAN = small area, owned; WAN = large area, leased infrastructure.
- Performance depends on bandwidth, users, media, interference and errors.
- Client-server: central control, costly, single point of failure. Peer-to-peer: cheap, simple, hard to manage.
- Hardware: NIC, switch, router, WAP, cables.
- DNS converts domain names to IP addresses; hosting and the cloud provide remote services.
- Star: switch is a single point of failure. Mesh: many routes, reliable, expensive.
- IP addresses route between networks; MAC addresses identify NICs.
- Protocols: TCP/IP, HTTP, HTTPS, FTP, POP, IMAP, SMTP. Layers separate jobs and support interoperability.