The big idea: TCP/IP splits networking into four layers. Each solves one problem and trusts the layer below to solve its own.
That separation is why the internet works at all: a web browser knows nothing about Wi-Fi, and Wi-Fi knows nothing about web pages.
Data travelling down the layers as it is sent, and back up as it is received.
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| Layer | Question it answers | Protocols | Unit |
|---|---|---|---|
| Application | What does this data mean? | HTTP, HTTPS, DHCP, DNS | Data |
| Transport | Did it all arrive, in order? | TCP, UDP | Segment |
| Internet | Which network, and which host? | IP | Packet |
| Link | How does it cross this cable or radio? | Ethernet, Wi-Fi | Frame |
Encapsulation: Going down, each layer wraps what it was given in its own header. The transport layer adds port numbers, the internet layer adds IP addresses, the link layer adds MAC addresses.
Going up at the far end, each layer removes its own header and passes the rest to the layer above. Neither end's application ever sees the others.
TCP against UDP, in one line: TCP guarantees delivery and order, retransmitting what is lost — at the cost of waiting. UDP sends and does not check.
A file transfer needs TCP. A live video call uses UDP, because a frame arriving late is worse than one never arriving at all.
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What layering buys
- Change Wi-Fi for Ethernet and no application changes
- A new protocol slots in at one layer only
- Faults can be located: which layer failed?
- Different vendors' equipment interoperates
Addressing at two levels
- IP address — which host, anywhere in the world
- MAC address — which device on this link
- The IP stays the same end to end
- The MAC changes at every hop
The cost
- Every layer adds a header, so overhead per packet
- A small payload can be mostly headers
- And a strict boundary sometimes prevents an optimisation
The addressing point is the examinable one: The IP address is unchanged from source to destination. The MAC address is rewritten at every router, because it only identifies the next device on the current link.
Answers that treat the two as interchangeable lose the mark.
How this is tested — you must name the layers in order and say what each contributes to one journey. It comes up two ways:
Paper 1 Section A
- Describe the function of the TCP/IP model, 3-4 marks
- Name the four layers
- State which layer a protocol belongs to
Paper 1 Section B — case study
- Trace data down and up the layers
- Explain why layering helps
The classic trap: Confusing IP and MAC addresses. The IP address is the same from end to end; the MAC address is rewritten at every hop, because it names only the next device on this link.
Describe what happens at each TCP/IP layer when a browser requests a web page, and explain one benefit of the layered design.
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