Network Protocols: TCP/IP, HTTP and Email

A network protocol is a set of rules that defines how data is formatted, transmitted, and received across a network.

Protocols ensure that devices from different manufacturers, running different operating systems, can communicate — because they all follow the same agreed rules. Without protocols, two devices attempting to communicate would produce incompatible data, like two people speaking different languages with no shared vocabulary.

Without protocols	With protocols
Devices interpret data differently	All devices parse data according to the same structure
Format mismatches cause errors	Standard structure prevents misinterpretation
No agreed error-handling	Defined rules for retransmission and acknowledgement
Incompatible systems cannot connect	Any standards-compliant device can participate

AQA 8525 requires knowledge of six protocols: TCP, IP, HTTP, HTTPS, SMTP, and IMAP. Each handles a distinct aspect of network communication. These six protocols are assessed by name — know what each one does.

TCP (Transmission Control Protocol) is responsible for breaking data into packets, numbering them, and ensuring all packets are received correctly at the destination.

Data sent over a network is not transmitted as one continuous block. It is divided into smaller packets that may travel by different routes and arrive out of order. TCP handles the reassembly.

Key TCP responsibilities:

• Divides data into numbered packets for transmission
• Requires the receiver to send an acknowledgement (ACK) for each packet received
• Requests retransmission of any packet that is lost, delayed, or arrives corrupted
• Reassembles packets into the correct order at the destination

Feature	TCP behaviour
Reliable delivery	Yes — every packet is acknowledged
Correct order	Yes — packets are reassembled in sequence
Error detection	Yes — corrupted or missing packets are re-sent
Use case	Web browsing, email, file download (accuracy required)

TCP trades speed for reliability. Every packet is accounted for through acknowledgements, which adds overhead. This is acceptable for web pages and email where every byte must arrive intact.

IP (Internet Protocol) handles addressing and routing — giving each device a unique address and directing packets towards the correct destination across the internet.

Every device connected to the internet has an IP address: a numerical label that identifies it on the network. When a packet is sent, IP reads the destination address and routes the packet hop by hop through the network infrastructure until it reaches the target.

IP concept	What it means
IP address	Unique identifier for a device on a network
Routing	Choosing the path from source to destination through intermediate nodes
Best-effort delivery	IP routes packets but does not guarantee arrival — that is TCP's job

IPv4 address format — four numbers from 0 to 255, separated by dots: 192.168.1.1. The total IPv4 address space is approximately 4 billion addresses, which has been exhausted; this is driving the transition to IPv6.

TCP and IP work together: IP routes packets to the right device; TCP ensures all packets arrive and are reassembled correctly. Together they form the foundation of internet communication.

(Extra context — not required by AQA 8525) IPv6 uses 128-bit addresses in hexadecimal groups, e.g. 2001:0db8::8a2e:0370:7334, providing a vastly larger address space than IPv4.

HTTP (HyperText Transfer Protocol) defines how web browsers request pages from servers and how servers respond. When a URL is entered in a browser, HTTP governs the request ("send me this page") and the server's response ("here is the HTML and associated content").

HTTPS (HyperText Transfer Protocol Secure) adds encryption to HTTP. The connection between the browser and the server is encrypted, so intercepted traffic cannot be read.

Feature	HTTP	HTTPS
Purpose	Transfer web page content	Transfer web page content securely
Encryption	None — plain text	Yes — encrypted in transit
Use case	Non-sensitive content	Login pages, banking, any form handling personal data
Browser indicator	http://	https:// and a padlock icon

Why HTTPS matters: an HTTP connection transmits all data — including passwords and personal information submitted in forms — as readable plain text. Anyone able to intercept the traffic (on the same Wi-Fi network, for example) can read it. HTTPS encrypts this data so only the intended server can decrypt it.

All modern browsers mark plain HTTP sites as "Not Secure" for pages that handle user input.

Email uses two separate protocols for the two stages of delivery: sending and receiving.

SMTP (Simple Mail Transfer Protocol) handles sending email. When a user clicks Send, their email client uses SMTP to transmit the message to the outgoing mail server. That server then uses SMTP to forward the message onward to the recipient's mail server.

IMAP (Internet Message Access Protocol) handles retrieving email from a mail server to a client. IMAP keeps messages stored on the server as the primary copy and synchronises them to any device that connects — phone, laptop, or webmail. The same inbox appears consistently across all devices.

Protocol	Direction	What it handles
SMTP	Sending	Transmits email from client to server and between servers
IMAP	Receiving	Retrieves and synchronises email from the server; server remains the primary store

Worked example — sending an email from alice@example.com to bob@otherdomain.com:

1. Alice's email client transmits the message to her outgoing mail server using SMTP
2. Her outgoing server forwards the message to Bob's mail server using SMTP
3. Bob's email client downloads and synchronises the message from his mail server using IMAP

Memory aid: SMTP = Send. IMAP = retrieve from the server to your device.

The TCP/IP model describes how network communication is divided into four layers. Each layer has a specific role. Data passes down through the layers on the sender's side and back up through them on the receiver's.

Layer	Name	Responsibility	Protocols / examples
4 (top)	Application	User-facing services; creating the data to be sent	HTTP, HTTPS, SMTP, IMAP
3	Transport	Breaking data into packets; reliable delivery	TCP
2	Internet	Addressing packets and routing them across networks	IP
1 (bottom)	Link	Physical transmission of bits over the hardware	Ethernet, Wi-Fi

How it works — sending a web request (top to bottom):

1. Application layer: the browser creates an HTTP GET request for a web page
2. Transport layer: TCP breaks the request into numbered packets
3. Internet layer: IP adds source and destination addresses to each packet
4. Link layer: the network interface card transmits the bits over the cable or wireless signal

At the receiving server, the process reverses: link layer passes data up to the internet layer (IP), which passes it to the transport layer (TCP for reassembly), which delivers the complete request to the application layer (the web server software).

1. Confusing TCP and IP

TCP and IP do different things. IP routes packets to the right device (addressing and routing). TCP ensures all packets arrive and are reassembled in the correct order (reliable delivery). They work together but are not interchangeable.

2. Saying HTTP and HTTPS are completely different protocols

HTTPS is HTTP with an encryption layer (TLS) added. The request-response model is identical — HTTPS adds security without changing the fundamental mechanism.

3. Reversing SMTP and IMAP

SMTP is for sending email. IMAP is for receiving (retrieving from the server). Reversing them is one of the most common errors on this topic. Mnemonic: SMTP = Send.

4. Claiming IP guarantees delivery

IP is best-effort — it routes packets towards the destination but makes no guarantee they arrive. TCP provides the delivery guarantee through acknowledgements and retransmission. If a question asks which protocol ensures reliable delivery, the answer is TCP.

5. Placing protocols in the wrong TCP/IP layer

HTTP, HTTPS, SMTP, and IMAP operate at the application layer (layer 4, top). TCP is at the transport layer (layer 3). IP is at the internet layer (layer 2). Placing any of these at the wrong layer in a question about the TCP/IP model will lose marks.