Networks - Lesson 5
Free Theory Networks Lesson 5
Lesson 5 of 6 Networks Series

Protocols & Layers

Learn the application protocols that power the internet - HTTP, HTTPS, FTP, SMTP, IMAP and POP3 - and understand the TCP/IP four-layer model that explains how they all work together.

GCSE and A-Level 6 sections Protocol sorter + layer explorer
Section 1

What is a Protocol?

In 2019, two satellites built by different agencies in different countries successfully docked and transferred data in orbit. Neither engineering team had met the other. Yet the communication worked perfectly. Why? Because both followed the same protocols - agreed sets of rules for how data would be formatted, transmitted and acknowledged.

Think about it: When your phone sends an email, that email might cross 12 different networks operated by 12 different companies on 3 different continents, and still arrive intact and readable. What has to be agreed in advance to make this possible?

A protocol is a set of rules that govern how data is formatted, transmitted and received over a network. Protocols ensure that devices from different manufacturers, running different operating systems, can communicate reliably. Without agreed protocols, the internet would not exist.

Protocols are organised into layers. Each layer handles a specific aspect of communication and relies on the layer below it. This layered design means that a change to one layer (for example, upgrading from Wi-Fi to Ethernet) does not affect the layers above it - your email protocols don't care whether the physical connection is wired or wireless.

Protocol
A set of agreed rules defining how data is formatted, sent, received and acknowledged between devices.
TCP
Transmission Control Protocol. Breaks data into packets, ensures reliable delivery and reassembles them in order.
Port number
A number (0-65535) identifying which application or service on a device should receive the incoming data.
Standard
An agreed, published specification that defines how a protocol must work, allowing different vendors to build compatible products.
Section 2

Application Layer Protocols

Application protocols define how specific types of data are communicated between applications. You need to know six protocols for GCSE - their purpose, how they differ from each other, and which port numbers they use.

HTTP
HyperText Transfer Protocol
The protocol used to request and send web pages between a client (browser) and a web server. Data is transmitted unencrypted - anyone intercepting the connection can read the data.
Port 80
Used for: Requesting web pages. The browser sends an HTTP GET request; the server responds with HTML content.
Exam note: HTTP is insecure because data is sent as plain text. This is why you should never enter passwords or payment details on HTTP sites. Modern browsers warn users when accessing HTTP pages.
HTTPS
HTTP Secure (HTTP + TLS)
HTTP with TLS (Transport Layer Security) encryption added. All data transmitted between browser and server is encrypted - intercepted data appears as unreadable ciphertext.
Port 443
Used for: All sensitive web communication - banking, shopping, login pages, any website handling personal data.
Exam note: HTTPS requires the server to have a valid TLS certificate (issued by a Certificate Authority). The padlock icon in the browser address bar indicates HTTPS is in use. Explain HTTPS as "HTTP with encryption" - specifically TLS/SSL encryption - not just "a more secure version".
FTP
File Transfer Protocol
A protocol used to transfer files between a client and a server over a network. FTP uses two separate connections: one for commands and one for data transfer.
Ports 20 & 21
Used for: Uploading website files to a web server; downloading large files from servers; transferring files between systems.
Exam note: Like HTTP, standard FTP is unencrypted (insecure). SFTP (SSH File Transfer Protocol) or FTPS (FTP Secure) add encryption. FTP uses two ports: port 21 for control/commands and port 20 for active data transfer.
SMTP
Simple Mail Transfer Protocol
The protocol used to send emails from a mail client to a mail server, and between mail servers. SMTP is for sending only - receiving requires IMAP or POP3.
Port 25 (server-to-server), Port 587 (client submission)
Used for: Sending emails. When you press Send, your email client uses SMTP to deliver the message to your mail server, which uses SMTP to forward it to the recipient's mail server.
Exam note: SMTP only sends - it cannot be used to retrieve emails. For receiving, IMAP or POP3 must be used. This distinction is a very common exam question. Remember: SMTP = Send.
IMAP
Internet Message Access Protocol
A protocol for receiving and managing emails. Emails remain stored on the server - the client only downloads a copy to view. Multiple devices can access the same inbox simultaneously.
Port 143 (unsecured), Port 993 (IMAP over TLS)
Used for: Accessing email on multiple devices (phone, laptop, tablet) with a synchronised inbox. Reading an email on your phone marks it as read on all devices.
IMAP vs POP3: IMAP keeps emails on the server and synchronises across all devices. POP3 downloads and usually deletes emails from the server. IMAP is better for users with multiple devices; POP3 is better for users with one device who need offline access.
POP3
Post Office Protocol version 3
A protocol for receiving emails. POP3 downloads emails from the server to the local device, typically deleting them from the server afterwards. Only one device sees the mail.
Port 110 (unsecured), Port 995 (POP3 over TLS)
Used for: Downloading email to a single device for offline access. Once downloaded, the email exists on your device only (the server copy is deleted).
POP3 vs IMAP: POP3 = downloads and deletes from server, one device only, no sync. IMAP = keeps on server, syncs across all devices, requires internet connection to view older mail not yet downloaded.
Port numbers you must know

HTTP: 80 | HTTPS: 443 | FTP: 20/21 | SMTP: 25/587 | IMAP: 143/993 | POP3: 110/995
Port numbers identify which service a packet is for. A router forwards a packet to the right application based on its destination port number. Port 443 = HTTPS traffic; Port 80 = HTTP traffic.

Section 3

Protocol Sorter

For each scenario below, identify which protocol is most appropriate. This mirrors the style of scenario-based exam questions for this topic.

Match the Protocol
Click the correct protocol for each scenario. You'll get immediate feedback with the reasoning.
Section 4

The TCP/IP Four-Layer Model

The TCP/IP model organises all networking protocols into four layers. Each layer has a specific responsibility. Data passes down the layers on the sender's device and back up the layers on the receiver's device. Click each layer to see full detail.

4
Application
HTTP, HTTPS, FTP, SMTP, IMAP, POP3, DNS
Provides services directly to user applications. Handles how data is presented and formatted for end users.
The application layer is where user-facing protocols live. When you browse the web, your browser uses HTTP/HTTPS at this layer. When you send email, your mail client uses SMTP. When you receive email, it uses IMAP or POP3. DNS also operates at this layer. This is the only layer that interacts directly with the user's application software.
3
Transport
TCP, UDP
Breaks data into segments/packets, manages reliable delivery, error checking and reassembly.
TCP (Transmission Control Protocol) is the main transport layer protocol. It breaks data into segments, numbers them, ensures reliable delivery by requiring acknowledgement of each segment, and reassembles them in order at the destination. If a packet is lost, TCP requests retransmission. UDP is an alternative that doesn't guarantee delivery but is faster - used for video streaming and gaming where speed matters more than perfect reliability.
2
Internet
IP (IPv4, IPv6), ICMP
Handles IP addressing and routing of packets across networks.
The internet layer adds IP addresses to packets (source and destination). Routers operate at this layer - they read the destination IP address and decide the best path to forward the packet. This layer does not guarantee delivery or order - that is TCP's job above it. The internet layer simply handles getting packets from one network to another.
1
Network Access
Ethernet, Wi-Fi, Bluetooth, MAC addressing
Handles physical transmission of data on the local network. MAC addresses, cables, signals.
The network access layer (sometimes split into Data Link and Physical in the OSI model) handles how data is physically transmitted across the local network. It manages MAC addresses (used by switches to deliver frames within a LAN), the actual electrical/optical/radio signals on cables or through the air, and frame formatting. Switches and NICs operate at this layer. This layer deals with hardware.
Why use a layered model?

A layered model offers three key advantages: (1) Modular design - each layer can be modified independently without affecting other layers. Upgrading from Wi-Fi 5 to Wi-Fi 6 only affects the network access layer. (2) Interoperability - different manufacturers can build products for specific layers, all working together as long as they follow the same standards. (3) Troubleshooting - problems can be isolated to a specific layer, making network issues easier to diagnose.

Section 5

Classify the Protocols

Layer Classification Activity
Drag each protocol or item to its correct TCP/IP layer.
HTTP
TCP
IP addressing
MAC addresses
SMTP
Breaks data into segments
Routing between networks
Physical signal transmission
DNS
Ethernet / Wi-Fi
Application Layer
Transport Layer
Internet Layer
Network Access Layer
Interactive Tool

Encapsulation Visualiser

Type a message and watch it travel down the TCP/IP layers, gaining a header (and footer) at each stage. Click Send to animate, then Receive to see de-encapsulation strip the headers away.

Interactive Tool

Port Number Rapid-Fire Quiz

A protocol name flashes up. Type the correct port number as fast as you can. 10 rounds, 10 seconds each. Port numbers come up in nearly every networks paper.

Ready to test your port knowledge?

10 protocols, 10 seconds each. Type the port number and press Enter or click Submit. Partial credit is not given.

Section 6

Quiz & Extended Thinking

Lesson Quiz - 5 questions
Protocol identification and TCP/IP model questions.
Question 1 of 5
A user wants to download their emails to their laptop and then read them on a plane with no internet connection. The emails should not remain on the server after downloading. Which email protocol should they use?
Question 2 of 5
What is the key difference between HTTP and HTTPS?
Question 3 of 5
Which protocol would a web developer use to upload their finished website files from their computer to the web server?
Question 4 of 5
Which layer of the TCP/IP model is responsible for breaking data into packets, ensuring reliable delivery and reassembling them in the correct order?
Question 5 of 5
Give one advantage of using a layered model for network protocols.
0
/5
Extended thinking

1. A user checks their email on both their phone and their laptop. Explain why IMAP is more appropriate than POP3 in this situation.

IMAP keeps emails on the server: With IMAP, emails are stored on the mail server, not downloaded to any single device. The phone and laptop each connect to the server and see the same inbox.

Synchronisation across devices: Any action (reading, deleting, moving to a folder) is reflected immediately on all devices. Reading an email on the phone marks it as read on the laptop too.

Why POP3 would fail here: POP3 downloads emails to one device and typically removes them from the server. If the user's phone downloads all emails via POP3, the laptop would show an empty inbox. The user would miss emails on one device.

Conclusion: IMAP is appropriate because the user needs a consistent, synchronised view of their inbox on multiple devices. POP3 is only suitable when a user has a single device and wants offline access to downloaded mail.
Extended thinking

2. Describe the role of the Transport layer in the TCP/IP model, explaining why this layer is essential for reliable data communication.

Role of the Transport layer: The transport layer (using TCP) is responsible for reliable end-to-end communication between applications on different devices.

Specific functions:
1. Segmentation: TCP breaks large data streams from the application layer into smaller segments (packets) of manageable size.
2. Sequence numbering: Each segment is numbered so the receiving device can reassemble them in the correct order, even if they arrive out of sequence.
3. Error checking: TCP includes checksums so the receiver can verify each segment arrived without corruption.
4. Acknowledgement and retransmission: The receiver sends acknowledgements (ACKs) for each segment. If no ACK is received, TCP automatically retransmits the missing segment.
5. Port numbers: TCP identifies which application on the destination device should receive the data using port numbers (e.g. port 443 for HTTPS, port 25 for SMTP).

Why it is essential: The internet layer (IP) provides best-effort delivery - it makes no guarantees. Without the transport layer, packets could be lost, arrive corrupted, or arrive out of order with no mechanism for correction. TCP compensates for the unreliability of IP, providing reliable transmission on top of an unreliable network.
Printable Worksheets

Practice what you've learned

Three levels covering protocols and the TCP/IP model.

Recall
Protocols Reference
Name the protocol from its description, match port numbers, complete the TCP/IP layer table.
Download
Apply
Scenario Protocol Selection
Given user scenarios, select and justify the most appropriate protocol. TCP/IP layer mapping tasks.
Download
Exam Style
Extended Answers
IMAP vs POP3 comparison, layered model advantages, TCP reliability mechanism questions.
Download
Networks Flashcards
Review HTTP, SMTP, IMAP, FTP, POP3 and TCP/IP definitions.
Open Flashcards
Networks Lesson 5 - Teacher Resources
Protocols & Network Layers
Teacher mode (all pages)
Shows examiner notes on exam practice pages
Suggested starter (5 min)
Ask two students to stand at opposite sides of the room. Tell them: "You need to have a conversation, but you are not allowed to agree in advance on any rules about how to communicate - no language, no gestures, no code. Go." After a few seconds of confused silence: "That is what happens when two computers try to communicate without a protocol. A protocol is simply the agreed set of rules that makes communication possible." Immediately concrete and memorable - students often cite this analogy unprompted in exam answers.
Lesson objectives
1Define the term protocol and explain why agreed protocols are necessary for network communication.
2Describe the purpose of HTTP, HTTPS, FTP, SMTP, IMAP and POP3 with their default port numbers.
3Compare IMAP and POP3, explaining when each is more appropriate.
4Name the four layers of the TCP/IP model and state the role of each layer.
5Classify protocols and networking activities into the correct TCP/IP layer.
6State at least two advantages of using a layered model for network design.
Key vocabulary (board-ready)
Protocol
A set of agreed rules that governs how data is transmitted between devices. Without protocols, two devices cannot communicate even if they are physically connected.
HTTP / HTTPS
HyperText Transfer Protocol: used to request and receive web pages. HTTP port 80 (unencrypted). HTTPS port 443 (HTTP + TLS encryption). HTTPS is the standard for all modern websites handling sensitive data.
FTP (File Transfer Protocol)
Used to transfer files between a client and a server. Port 21. Commonly used by web developers to upload site files to a hosting server.
SMTP / IMAP / POP3
SMTP (port 25/587): sends emails. IMAP (port 143): retrieves emails, keeping them on the server - ideal for multiple devices. POP3 (port 110): downloads emails to one device, removing them from the server.
DNS
Domain Name System. Translates domain names into IP addresses. Port 53. Operates at the Application layer of the TCP/IP model.
TCP/IP model (4 layers)
Application (HTTP, HTTPS, FTP, SMTP, DNS) | Transport (TCP, UDP - end-to-end delivery, ports, segmentation) | Internet (IP - addressing and routing) | Network Access (physical + data link - hardware, MAC, signals).
Encapsulation
The process of wrapping data with protocol headers as it passes down the layers. Each layer adds its own header. The receiving device strips each header as the data passes back up the layers.
Port number
A number (0-65535) that identifies which application or service a packet is destined for on a device. For example: port 80 = HTTP, port 443 = HTTPS, port 25 = SMTP.
Suggested lesson plan (60 min)
0-5 min: Starter: "communicate without rules" activity. Define protocol. Establish that protocols are the reason any two devices on the internet can exchange data, regardless of manufacturer or operating system.
5-20 min: Application protocols. Work through each protocol using the interactive cards. As each is revealed, students fill in a reference table in their notes: protocol name, purpose, port number. Pause at SMTP/IMAP/POP3 - this trio is the most frequently examined and most confused.
20-30 min: Protocol Sorter tool. Students work in pairs, saying their reasoning aloud before selecting. After each answer, ask: "Why couldn't you use [wrong protocol] here?" The elimination reasoning is as valuable as the correct selection.
30-45 min: TCP/IP model. Draw the four layers on the board as a stack. Assign each protocol to its layer. Then trace a single HTTP request through all four layers as a class, discussing what header information is added at each stage (encapsulation).
45-55 min: Advantages of layered models. Students generate a list in pairs, then share. Push for three distinct advantages beyond "easier to understand": interoperability, independent development, standardisation, troubleshooting isolation.
55-60 min: Exit ticket. Students write SMTP, IMAP, POP3, HTTP, HTTPS, FTP and their port numbers from memory. Mark immediately using mini-whiteboards or peer checking.
Discussion prompts
Your school email system uses IMAP. A student switches it to POP3 to "make it faster". They check email on their phone in the morning and then open their laptop in the afternoon. Describe exactly what they will experience on the laptop and explain why.
HTTPS uses the same structure as HTTP but encrypts the content with TLS. A student argues that any website using HTTPS must be safe and trustworthy. Explain specifically what HTTPS does protect and what it does not protect, and why the student's reasoning is flawed.
The TCP/IP model has 4 layers; the older OSI model has 7. Both are used in industry. If you were designing a new networking textbook, which model would you teach, and why? What are the practical and educational trade-offs of each choice?
Port numbers allow multiple applications to use the network simultaneously on the same device. A student is simultaneously browsing the web, downloading a file via FTP, and receiving an email. On their device, three separate "conversations" are happening at once. How does the operating system use port numbers to keep these three data streams separate?
Common misconceptions
X"SMTP can also be used to receive emails" - SMTP is send-only. Sending an email uses SMTP. Receiving uses IMAP or POP3. This distinction appears in almost every protocols exam question. Students who confuse send/receive lose multiple marks.
X"HTTPS is a completely different protocol from HTTP" - HTTPS is HTTP with TLS encryption applied. The underlying structure is identical. TLS encrypts the data before it is transmitted over the existing HTTP framework. HTTPS is not a separate protocol invented from scratch.
X"The TCP/IP model and OSI model are the same" - they are not. OSI has 7 layers; TCP/IP has 4. The OSI Application/Presentation/Session layers are collapsed into the TCP/IP Application layer. The OSI Data Link and Physical layers are collapsed into TCP/IP Network Access.
X"Protocols are optional" - without agreed protocols, two devices cannot communicate at all. Neither device knows how to format, interpret, or acknowledge the other's data. Protocols are as essential as a shared language - you cannot have a conversation in English with someone who only speaks Japanese without a translator (protocol) agreed in advance.
Exit ticket questions
Name the protocol used to send an email and give its default port number.
[2 marks - SMTP, port 25 (accept 587)]
A student has three devices and wants their emails to appear on all of them. State which email receiving protocol they should use and explain why.
[3 marks - IMAP; stores emails on the server (1); all devices see the same inbox (1); reading/deleting is synchronised across all devices (1)]
State the layer of the TCP/IP model where the Internet Protocol (IP) operates and describe its responsibility at that layer.
[2 marks - Internet layer (1); handles IP addressing and routing of packets between different networks (1)]
State two advantages of using a layered model for designing network systems.
[2 marks - any two of: each layer can be developed/updated independently without affecting other layers; standardisation allows different manufacturers' equipment to work together; easier to identify and isolate faults in a specific layer; different teams can work on different layers simultaneously]
Homework idea
Students create a "Protocols Reference Card" - a single A4 sheet that summarises every protocol from this lesson. For each protocol: name, purpose, port number, TCP or UDP, and one real-world example of when it is used. They may use any format (table, mind map, flashcard-style). The card must fit on one side of A4. The constraint forces prioritisation and genuine understanding. Bring the card to the next lesson: it becomes a revision tool for the unit exam.
Classroom tips
The SMTP/IMAP/POP3 trio is the most commonly confused area of this topic. Spend extra time here. The mnemonic "SMTP = Send Mail To People" helps students remember the direction. Then: "IMAP = I get the Mail from Any Place" (stays on server, multi-device). "POP3 = Pulls Only, Permanently" (downloads and removes).
When teaching the TCP/IP layers, build the stack visually on the board and physically trace a web request through it. Write "HTTP request: GET /index.html" at the top, then add each header as you go down: Transport adds port numbers, Internet adds IP addresses, Network Access adds MAC addresses and converts to signals. Then reverse the process for the response.
Port numbers are often underweighted in teaching but appear frequently in exam questions. Create a quick quiz: show a port number (80, 443, 21, 25, 22) and students call out the protocol. Run it three times at increasing speed. Repetition transforms this into automatic knowledge.
When students struggle to articulate the advantages of layered models, ask them: "If Microsoft wants to upgrade Windows' network stack to support a new encryption standard, do they need to redesign the physical network cables too?" The obvious "no" answer leads directly to the key advantage: layer independence. Each layer can be upgraded without affecting others.
Resources
Worksheet 1 - Recall: protocols, layers, port numbers Worksheet 2 - Apply: trace a web request, protocol selection Worksheet 3 - Exam-style questions Networks series overview Networks flashcards Set as class homework (coming soon)