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.