5.1d Network protocols — Edexcel 4CP0

What is a Protocol?

A network protocol is a set of rules that defines how data is transmitted between devices on a network. Protocols ensure that devices from different manufacturers can communicate reliably.

The TCP/IP Model

The TCP/IP model is the standard framework for internet communication. It has four layers, each with a specific role:

Layer	Name	Role	Example protocols
4 (top)	Application	User-facing services — browsers, email, file transfer	HTTP, HTTPS, FTP, SMTP, DNS
3	Transport	Breaks data into packets; ensures reliable delivery	TCP, UDP
2	Internet	Addresses packets and routes them across networks	IP (IPv4, IPv6)
1 (bottom)	Network Access (Link)	Physical transmission over the medium	Ethernet, Wi-Fi

Key Protocols

Protocol	Full name	Purpose
HTTP	HyperText Transfer Protocol	Transfers web pages from server to browser (unencrypted)
HTTPS	HTTP Secure	Encrypted version of HTTP using TLS/SSL; used for secure sites
FTP	File Transfer Protocol	Transferring files between computers over a network
SMTP	Simple Mail Transfer Protocol	Sending email from client to server and between servers
IMAP/POP3	Internet Message Access Protocol / Post Office Protocol	Receiving/retrieving email from a server
DNS	Domain Name System	Translates domain names (e.g. cszone.co.uk) into IP addresses
TCP	Transmission Control Protocol	Reliable, ordered delivery of packets with error checking
IP	Internet Protocol	Addressing and routing packets across networks

IP Addresses

Every device on a network needs a unique IP address to identify it.

IPv4: 32-bit address written as four decimal numbers, e.g. 192.168.1.1 — supports ~4.3 billion addresses
IPv6: 128-bit address in hexadecimal, e.g. 2001:0db8:85a3::8a2e:0370:7334 — supports vastly more addresses (needed as IPv4 addresses ran out)

MAC Addresses

A MAC (Media Access Control) address is a unique hardware identifier built into every network interface card (NIC). It is a 48-bit address written as 12 hex digits, e.g. A4:C3:F0:85:AC:2D. MAC addresses operate at the network access (link) layer and are used for communication within a LAN, while IP addresses are used for communication between networks.

Packets and Packet Switching

When data is sent across a network, it is broken down into small units called packets. Each packet consists of:

Header — source IP address, destination IP address, packet number, total packet count
Payload — the actual data being transmitted (a chunk of the original message)
Trailer — error-checking data (e.g. checksum)

Packet switching is the method of sending these packets across a network:

Each packet may travel by a different route to the destination
Packets arrive potentially out of order and are reassembled in the correct sequence at the destination
If a packet is lost or corrupted (detected via checksum), it is retransmitted
This is efficient — multiple devices can share the same network links simultaneously

Mobile Internet Standards: 3G, 4G, 5G

Standard	Approximate speed	Key features
3G	~7 Mbps	First mobile broadband standard; enabled smartphones to browse the web and use apps
4G / LTE	~50–150 Mbps	High-speed mobile internet; enabled HD video streaming and fast app downloads
5G	~1–10 Gbps	Very low latency; enables IoT devices, autonomous vehicles, and ultra-HD streaming

📝 Exam Tip: Know the difference between HTTP and HTTPS — the S stands for Secure (encrypted with TLS). For any question about secure online transactions (banking, shopping), HTTPS is the correct answer. DNS translates human-readable domain names into IP addresses.

⚠️ Common Mistakes

Saying SMTP receives email — SMTP sends email; IMAP or POP3 is used to receive it
Confusing IP address with MAC address — IP addresses identify devices on a network; MAC addresses identify the physical hardware
Saying HTTP is secure — HTTP is unencrypted; HTTPS uses TLS/SSL encryption