1.3.2a Network Protocols (TCP/IP, HTTP, DNS & more)
OCR J277 · GCSE Computer Science · ~14 min read
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What is a Protocol?
A protocol is a set of rules that govern how data is transmitted and communicated over a network. Protocols ensure that devices from different manufacturers can communicate with each other — they define the format, timing, sequencing, and error checking of data.
Without agreed protocols, devices would not be able to understand each other's data, just as two people speaking different languages cannot communicate without a common language.
The TCP/IP Protocol Stack
The TCP/IP model is the fundamental model for internet communication. It has four layers, each with specific protocols:
Layer
Name
Function
Protocols
4
Application
Provides network services to applications (web browsing, email, file transfer)
HTTP, HTTPS, FTP, SMTP, DNS
3
Transport
Breaks data into packets; ensures reliable delivery; handles errors
TCP, UDP
2
Internet/Network
Logical addressing (IP addresses); routes packets across networks
IP
1
Link/Network Interface
Physical transmission of data on the local network
Ethernet, Wi-Fi
Data is encapsulated as it passes down through the layers — each layer adds a header with its own control information. At the receiving end, the process is reversed (de-encapsulation).
Key Protocols — OCR J277
HTTP
HyperText Transfer Protocol
Used for transmitting web pages over the internet. Operates at the Application layer. When you type a URL, your browser sends an HTTP request to the web server; the server responds with the web page. HTTP data is unencrypted — anyone intercepting it can read it.
HTTPS
HyperText Transfer Protocol Secure
The secure version of HTTP. Uses SSL/TLS encryption to protect data in transit. Essential for online banking, shopping, and any site requiring login credentials. You can identify HTTPS by the padlock icon in the browser address bar.
FTP
File Transfer Protocol
Used to transfer files between a client and a server over a network. Commonly used to upload files to web servers or download large files. Uses ports 20 and 21. FTP is unencrypted; SFTP (Secure FTP) adds encryption.
SMTP
Simple Mail Transfer Protocol
Used to send email messages between mail servers. When you send an email, your mail client uses SMTP to deliver it to the recipient's mail server. Uses port 25 (or 587 with encryption). IMAP/POP3 are used to receive email.
DNS
Domain Name System
Translates human-readable domain names (e.g. www.bbc.co.uk) into IP addresses (e.g. 151.101.64.81) that computers can use. DNS servers maintain a database of domain name to IP address mappings. Without DNS, users would have to memorise IP addresses to visit websites.
TCP
Transmission Control Protocol
A connection-oriented protocol at the Transport layer. Breaks data into packets, sends them, and ensures all arrive correctly and in order. If a packet is lost, TCP requests it to be resent. Slower but reliable. Used for web browsing, email, file transfers.
IP
Internet Protocol
Handles logical addressing — assigns IP addresses to devices and routes packets across networks. Each packet is sent independently and may take different routes. IP does not guarantee delivery — that's TCP's job.
How DNS Works — Step by Step
User types www.example.com in the browser
The computer checks its local DNS cache — if found, done
If not cached, it queries the DNS resolver (usually the ISP's DNS server)
The resolver queries the root DNS server → finds the .com name server
The .com name server directs to example.com's authoritative DNS server
The authoritative server returns the IP address
The browser connects to the server at that IP address using HTTP/HTTPS
Protocol Comparison Table
Protocol
Layer
Purpose
Port
HTTP
Application
Web page transfer (unencrypted)
80
HTTPS
Application
Secure web page transfer (SSL/TLS)
443
FTP
Application
File transfer
20/21
SMTP
Application
Send email
25/587
DNS
Application
Domain name → IP address
53
TCP
Transport
Reliable data delivery (connection-oriented)
—
IP
Internet
Logical addressing and routing
—
Exam tip: Know what each protocol does and which layer it belongs to. HTTP vs HTTPS is a frequent question — key difference is HTTPS uses SSL/TLS encryption. DNS question types: "What does DNS do?" → translates domain names to IP addresses. Know that TCP is connection-oriented and ensures reliable delivery. Don't confuse SMTP (sends email) with POP3/IMAP (receives email).
⚠️ Common Mistakes
Saying HTTP is secure — it is NOT; HTTPS is the secure version using SSL/TLS
Confusing DNS with DHCP — DNS = domain names to IP addresses; DHCP = automatically assigns IP addresses
Saying SMTP is for receiving email — SMTP is for SENDING; POP3/IMAP are for receiving
Saying TCP is fast — TCP is reliable but slower than UDP; UDP is faster but not guaranteed
Forgetting that IP addresses are what computers actually use — DNS just translates
✅ Notes completed!
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Video coming soon
What's in this video
• TCP/IP model: 4 layers explained with examples
• HTTP vs HTTPS: what SSL/TLS does
• DNS: step-by-step domain lookup animation
• FTP, SMTP: when and why you use them
Click slide or press arrow keys to navigate
Worksheet — 1.3.2a Network Protocols
8 questions · 22 marks
Q1What is a network protocol? Why are protocols necessary?[2]
✅ Mark scheme
A set of rules governing how data is transmitted between devices on a network [1]; protocols are necessary so that devices from different manufacturers can communicate (allows interoperability) [1].
Q2State the difference between HTTP and HTTPS.[2]
✅ Mark scheme
HTTP transmits web pages without encryption (data is unprotected/readable by others) [1]; HTTPS uses SSL/TLS encryption to protect data in transit (secure) [1].
Q3Describe what DNS does. Give an example to illustrate your answer.[3]
✅ Mark scheme
DNS (Domain Name System) translates domain names into IP addresses [1]; this is necessary because computers use IP addresses not domain names to communicate [1]; e.g. www.bbc.co.uk → 151.101.64.81 [1].
Q4What is FTP used for? Name one use case.[2]
✅ Mark scheme
FTP (File Transfer Protocol) is used to transfer files between a client and a server [1]; e.g. uploading website files to a web server / downloading large files from a server [1].
Q5Name the protocol used to send emails. What protocol is used to receive emails?[2]
✅ Mark scheme
SMTP (Simple Mail Transfer Protocol) is used to send emails [1]; POP3 or IMAP is used to receive emails [1].
Q6Describe the role of TCP in data transmission. Why is it described as "connection-oriented"?[3]
✅ Mark scheme
TCP (Transmission Control Protocol) breaks data into packets and ensures they all arrive at the destination correctly and in order [1]; if a packet is lost it is requested again (retransmission) [1]; "connection-oriented" means a connection is established before data is sent and maintained throughout the transmission (three-way handshake) [1].
Q7Describe the four layers of the TCP/IP model and state one protocol used at each layer.[4]
✅ Mark scheme
Application layer: user-facing services e.g. HTTP/HTTPS/FTP/SMTP/DNS [1]; Transport layer: breaks data into packets, reliable delivery e.g. TCP [1]; Internet/Network layer: logical addressing and routing e.g. IP [1]; Link/Network Interface layer: physical transmission on local network e.g. Ethernet/Wi-Fi [1].
Q8A user visits www.cszone.co.uk for the first time. Describe the process that occurs to resolve the domain name to an IP address.[4]
✅ Mark scheme
The browser checks the local DNS cache [1]; not found, so queries the DNS resolver (ISP's DNS server) [1]; the resolver queries root and then .co.uk name servers to find the authoritative DNS server for cszone.co.uk [1]; the authoritative server returns the IP address [1]; browser connects to the web server at that IP using HTTP/HTTPS. Award 1 mark for each correct step (max 4).
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Mini Test — 1.3.2a Network Protocols
10 questions · 10 marks · 10 minutes
⏱ 10:00
10 marks
Section A — Multiple Choice [5 marks]
Q1What does HTTPS provide that HTTP does not?
Q2Which protocol translates domain names into IP addresses?
Q3Which protocol is used to send email?
Q4At which layer of the TCP/IP model does HTTP operate?
Q5Which protocol is described as "connection-oriented" and ensures reliable delivery?
Section B — Short Answer [5 marks]
Q6What does FTP stand for and what is it used for?
Mark schemeFile Transfer Protocol [½]; used to transfer files between a client and a server [½]. [1]
Q7Why do websites use HTTPS rather than HTTP for login pages?
Mark schemeHTTPS encrypts data using SSL/TLS, so passwords/personal data cannot be intercepted and read by others during transmission. HTTP sends data unencrypted. [1]
Q8Describe what happens when a user types www.bbc.co.uk and presses Enter (DNS step only).
Mark schemeThe browser queries a DNS server to translate the domain name www.bbc.co.uk into an IP address; the DNS server returns the IP address and the browser then connects to that address. [1]
Q9State the role of IP (Internet Protocol) in data transmission.
Mark schemeIP handles logical addressing (assigns IP addresses) and routes data packets across networks from source to destination. [1]
Q10State one protocol operating at the Transport layer of the TCP/IP model and describe its purpose.
Mark schemeTCP (Transmission Control Protocol) [½]; breaks data into packets, ensures reliable ordered delivery, retransmits lost packets [½]. [1]