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📖 Paper 1 · Topic 4: Computer Networks
4.3a Protocols & Layers (TCP/IP)
Edexcel 1CP2 · GCSE Computer Science · ~10 min read · 🔒 Pro
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⭐ Enrichment lesson — this topic is not assessed in the Edexcel 1CP2 GCSE exam. It provides valuable extra knowledge but should not replace revision of the core specification.

What is a Protocol?

A protocol is a set of rules that govern how data is transmitted between devices on a network. Protocols ensure that different devices and systems can communicate with each other even if they are manufactured by different companies.

Key protocols to know for Edexcel 1CP2:

ProtocolFull namePurpose
HTTPHyperText Transfer ProtocolTransfers web pages over the internet
HTTPSHTTP SecureEncrypted HTTP using TLS/SSL — secure web browsing
FTPFile Transfer ProtocolTransfers files between computers on a network
SMTPSimple Mail Transfer ProtocolSends emails from client to mail server
IMAPInternet Message Access ProtocolRetrieves emails — emails stored on server, accessible from multiple devices
POP3Post Office Protocol v3Retrieves emails — emails downloaded to device and deleted from server
TCP/IPTransmission Control Protocol / Internet ProtocolCore internet protocol — splits data into packets, routes and reassembles them
DNSDomain Name SystemTranslates domain names (e.g. google.com) into IP addresses

TCP/IP — The Four-Layer Model

The TCP/IP model describes how data is prepared, sent, and received across the internet. It has four layers, each with specific responsibilities:

LayerNameResponsibilityProtocols
4ApplicationProvides network services to applications; formats data for userHTTP, HTTPS, FTP, SMTP, IMAP, DNS
3TransportSplits data into packets; ensures reliable delivery; error checkingTCP, UDP
2InternetAdds IP addresses; routes packets across networksIP
1Network Access / LinkHandles physical transmission of data — hardware, cables, signalsEthernet, Wi-Fi

How the Layers Work — Encapsulation

When data is sent, each layer encapsulates the data from the layer above by wrapping it with a header (and sometimes a footer) containing relevant information:

  1. Application layer — data is created (e.g. a webpage request)
  2. Transport layer — adds port numbers and sequence numbers; splits into segments
  3. Internet layer — adds source and destination IP addresses → becomes a packet
  4. Network Access layer — adds MAC addresses and transmits as frames on the physical medium

At the receiving end, each layer strips off its header — de-encapsulation — until the original data is delivered to the application.

TCP vs UDP

FeatureTCPUDP
Full nameTransmission Control ProtocolUser Datagram Protocol
ConnectionConnection-oriented (handshake first)Connectionless — sends without handshake
ReliabilityReliable — acknowledges receipt, retransmits lost packetsUnreliable — no acknowledgement or retransmission
SpeedSlower (overhead of acknowledgements)Faster (no overhead)
OrderPackets reassembled in correct orderNo guaranteed order
Use casesWeb browsing, email, file transferLive video streaming, online gaming, VoIP

IP Addresses

An IP address (Internet Protocol address) is a unique numerical label assigned to every device on a network, used to identify and locate it. There are two versions:

  • IPv4 — 32 bits, written as four groups of numbers separated by dots (e.g. 192.168.1.1). Provides ~4.3 billion addresses
  • IPv6 — 128 bits, written in hexadecimal (e.g. 2001:0db8:85a3::8a2e:0370:7334). Provides vastly more addresses to cope with the growth of internet-connected devices
Exam tip: Be able to name each TCP/IP layer and its role. For 4-mark "explain" questions: name the protocol → state what layer it operates at → explain its specific function. E.g. "HTTPS operates at the Application layer; it sends encrypted web pages using TLS, protecting data from interception."
⚠️ Common Mistakes
  • Confusing SMTP (sends email) with IMAP/POP3 (retrieves email)
  • Saying TCP and UDP do the same thing — examiners look for the reliability difference
  • Forgetting DNS — it's the protocol that translates domain names to IP addresses
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✍️ Worksheet — 4.3a Protocols & Layers

8 questions · TCP/IP model and key protocols

Q1State what is meant by a 'protocol' in computer networking.[2]
✅ Mark scheme
A set of rules [1] that govern how data is transmitted/communicated between devices on a network [1]. Protocols allow devices from different manufacturers to communicate with each other.
Q2Name the FOUR layers of the TCP/IP model in order (starting from the highest layer).[4]
✅ Mark scheme
Application layer [1]; Transport layer [1]; Internet layer [1]; Network Access / Link layer [1]. One mark for each correct layer in the correct order.
Q3Describe the role of the Transport layer in the TCP/IP model.[3]
✅ Mark scheme
Splits data into packets/segments [1]; adds port numbers and sequence numbers [1]; ensures reliable delivery — acknowledges receipt and requests retransmission of lost packets (using TCP) [1].
Q4Give TWO differences between TCP and UDP.[4]
✅ Mark scheme
TCP is connection-oriented (uses a handshake), UDP is connectionless [2]; TCP is reliable — acknowledges packets and retransmits if lost, UDP does not [2]; TCP is slower due to acknowledgement overhead, UDP is faster [2]. Any two differences, one mark each side.
Q5State the role of DNS in internet communication.[2]
✅ Mark scheme
DNS (Domain Name System) translates/converts human-readable domain names (e.g. www.google.com) [1] into their corresponding numerical IP addresses [1], so that devices can locate each other on the internet.
Q6Explain the difference between HTTP and HTTPS.[3]
✅ Mark scheme
HTTP (HyperText Transfer Protocol) sends web pages between browser and server [1]; HTTPS (HTTP Secure) does the same but encrypts the data using TLS/SSL [1]; HTTPS prevents eavesdropping and is essential for secure transactions (e.g. online banking, shopping) [1].
Q7Explain what is meant by 'encapsulation' in the TCP/IP model.[3]
✅ Mark scheme
Encapsulation is the process of each layer wrapping data from the layer above with a header (and sometimes trailer) [1] containing control information such as addresses, sequence numbers, and error-checking data [1]; at the receiving end, each layer strips its header (de-encapsulation) to pass data up to the next layer [1].
Q8Give ONE advantage of IPv6 over IPv4.[2]
✅ Mark scheme
IPv6 uses 128-bit addresses compared to IPv4's 32-bit addresses [1]; this provides a vastly larger number of possible addresses (2^128 vs 2^32 ≈ 4.3 billion) — necessary due to the huge number of internet-connected devices (IoT, smartphones) and exhaustion of IPv4 addresses [1].
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Mini Test — Protocols & Layers

10 minutes · Exam-style

← 4.2 Network HardwareTopic 4 · Networks4.3b The Internet →