A network protocol is a set of rules that governs how data is formatted, transmitted, and received between devices on a network. Protocols allow devices made by different manufacturers and running different software to communicate reliably — in the same way that a shared language allows people from different countries to understand one another.
Why do protocols exist?
Without agreed protocols, a laptop from one manufacturer would be unable to exchange data with a server made by another, even if both were physically connected. Protocols define precisely:
- How data is broken into packets.
- What each packet's header must contain (source address, destination address, sequence number, etc.).
- How errors are detected and handled.
- How the connection is established and ended.
The GCSE Computer Science specifications (AQA, OCR, Edexcel) require students to name and describe the key protocols and explain the layered model in which they operate.
What is the TCP/IP model?
The TCP/IP model organises protocols into four layers. Each layer has a specific responsibility and communicates only with the layer immediately above and below it. This layered approach means a change in one layer (say, upgrading physical hardware) does not require changes in other layers.
| Layer | Name | Responsibility | Example protocols |
|---|---|---|---|
| 4 | Application | Provides network services directly to user applications | HTTP, HTTPS, FTP, SMTP, DNS |
| 3 | Transport | Breaks data into packets; ensures reliable delivery and correct ordering | TCP, UDP |
| 2 | Internet | Adds IP addresses; routes packets across networks | IP (IPv4, IPv6) |
| 1 | Network Access (Link) | Handles physical transmission on the local network | Ethernet, Wi-Fi (802.11) |
Data sent from an application travels down the layers (each adds a header), across the network, then up the layers at the receiving end (each strips its header). This process is called encapsulation (adding) and decapsulation (removing).
How does TCP ensure reliable delivery?
TCP (Transmission Control Protocol) provides a reliable, ordered, error-checked stream of data between two devices.
- Three-way handshake — before data is sent, the sender and receiver establish a connection: SYN → SYN-ACK → ACK.
- Sequencing — each packet is numbered. If packets arrive out of order (common on the internet), they are reassembled correctly.
- Acknowledgements (ACK) — the receiver sends an acknowledgement for every packet received. If the sender does not receive an ACK within a timeout period, it retransmits.
- Error checking — a checksum in each packet's header allows the receiver to detect corrupted data and request retransmission.
UDP (User Datagram Protocol) is the alternative: it sends packets without establishing a connection, without guaranteeing delivery, and without reordering. UDP is used when speed matters more than reliability — video streaming, online gaming, DNS lookups. A dropped video frame is less disruptive than the delay caused by requesting a retransmission.
What is IP addressing?
Every device on the internet has a unique IP address that identifies it. The currently dominant version, IPv4, uses 32-bit addresses written as four decimal numbers separated by dots:
192.168.1.1 — a common home router address.
IPv4 supports approximately 4.3 billion unique addresses — no longer enough. IPv6 uses 128-bit addresses written in hexadecimal:
2001:0db8:85a3:0000:0000:8a2e:0370:7334
IPv6 supports 3.4 × 10³⁸ unique addresses — effectively inexhaustible.
What does DNS do?
The Domain Name System (DNS) translates human-readable domain names (e.g. aitutors.me) into the IP addresses that computers actually use to route traffic.
How a DNS lookup works:
- You type
aitutors.mein your browser. - Your device checks its local DNS cache. If the IP is cached, it uses it directly.
- If not cached, your device queries a recursive resolver (usually provided by your ISP or a service like Google's 8.8.8.8).
- The resolver queries a root name server, then a TLD name server (for
.me), then the authoritative name server foraitutors.me. - The authoritative server returns the IP address.
- Your browser connects to that IP address.
Without DNS, you would need to memorise IP addresses for every website you visit. DNS is often called "the phone book of the internet."
Key application-layer protocols compared
| Protocol | Full name | Purpose | Port |
|---|---|---|---|
| HTTP | HyperText Transfer Protocol | Transfers web pages (unencrypted) | 80 |
| HTTPS | HTTP Secure | Transfers web pages over TLS encryption | 443 |
| FTP | File Transfer Protocol | Transfers files between client and server | 20/21 |
| SMTP | Simple Mail Transfer Protocol | Sends email from client to server | 25/587 |
| IMAP | Internet Message Access Protocol | Retrieves email from server (synchronised) | 143/993 |
| POP3 | Post Office Protocol v3 | Downloads email from server (deletes from server) | 110/995 |
| DNS | Domain Name System | Resolves domain names to IP addresses | 53 |
| SSH | Secure Shell | Secure remote command-line access | 22 |
Frequently asked questions
What is the difference between TCP and UDP?
TCP is a connection-oriented protocol: it establishes a connection with a handshake before sending data, numbers every packet, requires acknowledgement of receipt, and retransmits lost packets. This makes TCP reliable but slower. UDP is connectionless: it sends data without establishing a connection and without guaranteeing delivery. UDP is faster and is used where occasional lost data is acceptable — for example, live video streaming, DNS, and online gaming.
What is a port number?
A port number identifies a specific application or service on a device. An IP address routes data to the correct device; the port number routes it to the correct application on that device. For example, a web server listens on port 80 (HTTP) or 443 (HTTPS). When you access a website, your browser connects to the server's IP address at port 443. Your own connection is assigned a random high-numbered "ephemeral" port so the reply can find its way back to the correct browser tab.
Why do we still use IPv4 if we are running out of addresses?
Network Address Translation (NAT) has extended IPv4's lifespan by allowing many devices on a private home or office network to share a single public IPv4 address. The router holds the public IP; every device behind it gets a private address (in the 192.168.x.x range, for example). NAT means a household with ten devices only needs one public IPv4 address. IPv6 adoption is growing but the transition has been slow because it requires updating infrastructure throughout the internet.
What happens if a DNS server goes down?
If the DNS server used by your ISP goes down and your device's DNS cache has expired, you cannot resolve domain names. Websites would appear unreachable even though the internet connection itself is working. You might still reach sites by typing their IP address directly. Using a secondary DNS server (most systems are configured with a backup) provides resilience. Organisations like Cloudflare (1.1.1.1) and Google (8.8.8.8) operate public DNS resolvers as fast, reliable alternatives to ISP DNS servers.
For Socratic GCSE Computer Science tutoring on networks, protocols, and beyond, visit aitutors.me.