Network Protocols and the TCP/IP Model
Why Protocols Are Necessary
Every time a web page loads or an email arrives, devices made by different manufacturers, running different operating systems, exchange data seamlessly. This works because of network protocol — agreed sets of rules that all devices follow when communicating. Without protocols, a phone made by one company could not send a message to a laptop made by another. Protocols create a common language for devices that would otherwise have nothing in common.
Protocols define many aspects of communication: how data is broken into chunks (packets), how packets are addressed, how errors are detected, and how the two ends of a connection signal that they have finished. Different tasks require different protocols — browsing the web uses different rules from sending an email, which uses different rules from transferring a large file. This is why several protocols coexist and each has its own specific purpose.
The protocols you must know: Ethernet (wired LAN data transmission), Wi-Fi (wireless LAN data transmission), TCP/IP (reliable internet transmission), HTTP (web page transfer), HTTPS (encrypted web transfer), FTP (file transfer between computers), SMTP (sending email), POP3 (retrieving email — downloads and deletes from server), IMAP (retrieving email — keeps on server, syncs across devices).
Protocols in Detail
Ethernet is the standard protocol for wired LANs. It defines the rules for transmitting data across a physical cable and uses MAC addresses (hardware addresses built into each network interface card) to identify devices on the same local network segment. When your computer sends data to another device on the same LAN, Ethernet handles the local delivery.
Wi-Fi is the wireless equivalent of Ethernet. It governs how devices communicate over radio waves within a local area. Like Ethernet, Wi-Fi uses MAC addresses to direct data on the local network. The key differences are the transmission medium (radio waves instead of cable) and the added challenge of managing shared radio spectrum fairly among multiple devices.
TCP/IP is really two protocols working together. IP (Internet Protocol) assigns addresses to devices and routes packets across networks. TCP (Transmission Control Protocol) sets up the connection, splits data into numbered packets, checks they all arrive in order, and requests retransmission of any packet that is lost or corrupted. Together they form the backbone of internet communication.
HTTP (Hypertext Transfer Protocol) defines how a web browser requests a web page and how the server sends it back. When you type a web address, your browser sends an HTTP request; the server responds with the HTML content of the page. HTTP transmits data without encryption — the content can be read by anyone who intercepts the connection.
HTTPS (Hypertext Transfer Protocol Secure) adds encryption to HTTP. The data exchanged between browser and server is scrambled, so intercepting it reveals nothing useful. HTTPS is essential for any page that handles sensitive data — online banking, shopping checkouts, login forms. Look for the padlock icon in a browser’s address bar to confirm HTTPS is in use.
FTP (File Transfer Protocol) is designed specifically for moving files between a client and a server. Web developers use FTP to upload website files to a web hosting server; IT teams use it to distribute software updates across a network. FTP separates the control connection (commands) from the data connection (the file itself), running them on different network ports.
SMTP (Simple Mail Transfer Protocol) handles the sending of email. When you press Send, your email application uses SMTP to submit your message to your outgoing mail server. SMTP is also used between mail servers to relay messages from the sender’s server to the recipient’s server.
POP3 (Post Office Protocol version 3) retrieves emails from a mail server by downloading them to the local device. By default, messages are deleted from the server once downloaded — so if you read email on your phone using POP3, it will not appear on your laptop. POP3 suits users who only access email from one device.
IMAP (Internet Message Access Protocol) leaves emails on the server and synchronises the state across all devices. If you read an email on your phone, it is marked as read on your laptop too. If you delete it, it disappears everywhere. IMAP is the standard approach for anyone using email on multiple devices — phones, tablets, laptops and desktops all see the same inbox.
The key distinction to remember: SMTP sends; POP3 downloads and typically removes from server; IMAP keeps on server and syncs everywhere.
The Four-Layer TCP/IP Model
When data travels across the internet, the process is organised into four distinct layers. Each layer handles a specific job. Together, they form the TCP/IP model. Using layers keeps the design modular: a new application protocol can be added without rewriting how packets are routed; the physical hardware can change without changing how applications work.
The four layers of the TCP/IP model (top to bottom): Application — where applications operate using protocols such as HTTP, HTTPS, FTP, SMTP, POP3, IMAP. Transport — manages communication sessions and reliable delivery; TCP operates here. Internet — handles addressing and routing of packets; IP operates here. Link — the physical hardware layer; the network interface card (NIC), Ethernet and Wi-Fi operate here, using MAC addresses.
Application layer — this is where network applications such as web browsers, email clients and file transfer tools operate. The protocols at this layer — HTTP, HTTPS, FTP, SMTP, POP3 and IMAP — define how those applications format and exchange data. When you open a web browser, you are interacting with the Application layer.
Transport layer — manages the communication session between two hosts. TCP operates here: it breaks data into packets, numbers them, and verifies that all packets arrive in the correct order. If a packet is lost, TCP requests that it be sent again. This is what makes TCP a “reliable” protocol. The Transport layer also handles flow control (adjusting the rate of transmission to avoid overwhelming the receiver).
Internet layer — handles addressing and routing. IP operates here: it attaches source and destination IP addresses to each packet and determines the best path through the network of routers to reach the destination. Every router on the internet operates at this layer, reading the IP address and forwarding the packet accordingly.
Link layer — the lowest layer, responsible for the physical transmission of data across a single network link. The network interface card (NIC) sits here. Ethernet (for wired connections) and Wi-Fi (for wireless) operate at this layer, using MAC addresses to deliver data on the local network. A common exam error is confusing MAC addresses at the Link layer with IP addresses at the Internet layer — they operate at different layers and serve different purposes.
Data flow: when sending data, the application at the top passes its data down through all four layers (Application → Transport → Internet → Link), each layer adding its own addressing information. The data then travels physically across the network and is passed back up through all four layers on the receiving device (Link → Internet → Transport → Application), which strips away each layer’s information to deliver the original message to the application.