A2.1.5 · Network fundamentals
The TCP/IP model
By the end you can
Describe the four layers of the TCP/IP model, what each does, and how data is wrapped on the way down and unwrapped on the way up.
Sending a web page across the world involves hundreds of separate jobs: choosing a protocol, splitting the data, finding a route, turning bits into light or radio. Nobody designs that as one lump. The splits it into four layers, each with its own job and its own protocols. Each layer talks only to the one above and the one below it.
The four layers#
- Application layer (top): the protocols that applications use. A browser builds an HTTP request here; email uses SMTP; names are looked up with DNS; DHCP hands out addresses.
- Transport layer: gets data from one application to another. It splits the data into segments, adds source and destination port numbers, and with TCP adds sequence numbers, checksums and acknowledgements. TCP and UDP live here.
- Internet layer: gets a packet from one computer to another, across many networks. It adds the source and destination IP addresses (and a time to live, so packets cannot circle forever). IP lives here, and routers work here.
- Network interface layer (bottom): moves frames across one physical link. It adds MAC addresses and a checksum, and the NIC converts the frame to electrical, light or radio signals. Ethernet (IEEE 802.3) and Wi-Fi (802.11) live here, with switches and access points.
Some books call the bottom layer the “physical layer” or “link layer”. It is the same idea; use the name your syllabus uses. The older seven-layer OSI model describes the same ground in finer slices, and TCP/IP largely replaced it.
Wrapping and unwrapping#
On the way down, each layer wraps what it receives in its own header: this is . Your data becomes a segment, then a packet, then a frame. At the other end, each layer reads and removes its header and passes the rest up: decapsulation. Routers along the way open the packet only as far as the IP header, and switches only as far as the frame header; only the two ends open everything.
Going down (sender)
Data (Application)
Your data: “GET /index.html”
Size now: 15 bytes
Step 1 of 8
Your browser writes the message (application layer).
HTTP
TCP sequence numbers and acknowledgements
IP addresses and routers
MAC addresses and switches
Port numbers such as 80 and 443
DNS and DHCP
Converting a frame to light pulses in a fibre
0 of 7 matched
Check yourself
The TCP/IP model
Quick questions, made fresh every time and marked here. Do a few until they feel easy.
Exam practice
Paper 1-style questions
5 questions · 17 marks
Write your answer as you would in the exam, then show the mark points and tick each one your answer made. These are our own questions in the style of the exam, not past-paper questions.
1.Describe the function of each layer of the TCP/IP model.
[4]Describe: A detailed account: say what happens, step by step if it is a process.
2.Outline how the transport layer helps make data transmission reliable.
[2]Outline: A brief account: the main points, without much detail.
3.Describe the role of the internet layer.
[3]Describe: A detailed account: say what happens, step by step if it is a process.
4.Explain what happens to a web request as it passes down the layers on the sending computer.
[4]Explain: Give reasons: say what happens AND why, or how one thing causes another.
5.Explain why network communication is organised into layers.
[4]Explain: Give reasons: say what happens AND why, or how one thing causes another.
Past papers
Old exam questions on this topic
From HL Paper 1, May 2014 to November 2025. These papers were set on the previous syllabus, so the wording may differ from today's exam, but the topic is the same. Find the questions and markschemes in your school's past-paper pack.
| Paper | Question | Marks | About |
|---|---|---|---|
| May 2017 | 6(a) | 2 | Two layers of the OSI model (new course uses TCP/IP) |
| May 2015 | 10(a) | 3 | The layered OSI framework (new course uses TCP/IP) |
In a nutshell
- The TCP/IP model has four layers: application, transport, internet, network interface.
- Application: HTTP, HTTPS, SMTP, DNS, DHCP. Transport: TCP or UDP, ports, segments. Internet: IP addresses, packets, routing. Network interface: MAC addresses, frames, signals.
- Going down the sender’s stack each layer wraps the data with its own header (encapsulation); the receiver strips them in reverse order (decapsulation).
- Routers work at the internet layer; switches, NICs, modems and access points at the network interface layer.
- Layering splits a hard job into simple ones, lets layers be swapped independently, and makes faults easier to find.