Networking: Devices, Topologies & Protocols

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OSI and TCP/IP reference models

⏱ 4 min read🧩 5 question types🎯 15 practice Q

The idea in one minute

The OSI model splits network work into 7 layers, so each layer can be built and replaced on its own. The TCP/IP model has 4 layers, and the real Internet runs on it. Most exam questions ask the layer count, the order, or which layer does a given job.

Why a reference model exists

Sending data needs many jobs: signals, framing, addressing, delivery, dialogue control, translation and user services. The OSI (Open Systems Interconnection) model splits them into 7 layers. Each layer can be designed and replaced on its own.

The 7 layers, bottom to top

#	Layer	Job	Works with
1	Physical	raw bits on the medium	cables, hubs, repeaters
2	Data Link	frames, MAC addresses, error detection	Ethernet, switches, bridges, NIC
3	Network	logical addressing and routing	IP, routers
4	Transport	end-to-end delivery, ports, reliability	TCP, UDP
5	Session	opens, manages and closes dialogues	NetBIOS, RPC
6	Presentation	format, encryption, compression	JPEG, MP3, SSL/TLS
7	Application	user-facing network services	HTTP, FTP, SMTP, DNS

Tip: Bottom-up: "Please Do Not Throw Sausage Pizza Away". Top-down: "All People Seem To Need Data Processing".

Data units on the way down

Data changes name at each layer as it moves down the sender's stack.

Layer 4 makes segments.
Layer 3 makes packets.
Layer 2 makes frames.
Layer 1 sends bits.

The receiver does the reverse, from layer 1 up to layer 7.

How layer questions are asked

Device to layer: hub and repeater are layer 1. Switch and bridge are layer 2. Router is layer 3.
Job to layer: routing is Network (3). Framing and MAC are Data Link (2). Encryption and compression are Presentation (6).
Number: OSI has 7 layers. TCP/IP has 4.

Watch: Encryption is not Application. Routing is not Transport. Test the planted option against its neighbour layer.

Sorting a layer question

First decide what the question really asks. Then use the matching tool.

Question asks	What to do
a number	7 for OSI, 4 for TCP/IP
an order	use the mnemonic and count
a job	match the job to one layer
a worker (device or protocol)	use the address type or the job

Example: "Which layer delivers data end to end using ports?" The job is end-to-end delivery. That is Transport (4).

For job questions, compare the planted option with its neighbour. Transport only delivers after the Network layer has found the path. Application serves the user. Presentation prepares the data format for it.

The TCP/IP model

The real Internet runs on TCP/IP. OSI is the teaching map.

TCP/IP layer	OSI layers it covers
Application	5 + 6 + 7 (Session, Presentation, Application)
Transport	4
Internet	3 (Network)
Network Access / Link	1 + 2 (Physical, Data Link)

Rule: Which model has 4 layers? TCP/IP.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

OSI layer count and order

How to spot it:

How many layers does the OSI model have, an ordering question, or the layer directly above or below another.

Method

Recall the 7 layers from Physical (1) to Application (7).
Use a mnemonic to fix the order.
Transport is the 4th, the middle layer.
Count up or down from the layer given.

Why it works:

The order never changes, so a mnemonic is enough.

Try this

How many layers are there in the OSI reference model?

Show solution

OSI is the 7-layer model.
The 4-layer model is TCP/IP.

Answer

Type 2very common4 practice Q

Function-to-layer matching

How to spot it:

Routing is done at which layer, encryption or compression is a which-layer function, or framing and MAC addressing belong to which layer.

Method

Bits on cables: Physical (1).
Frames and MAC: Data Link (2). Routing: Network (3).
Delivery and ports: Transport (4). Dialogues: Session (5).
Encryption and compression: Presentation (6). User services: Application (7).

Why it works:

Each layer owns one clear job.

Try this

Encryption and data compression are functions of which OSI layer?

Show solution

Data format work sits below the Application.
That layer is Presentation.

Answer

Presentation (layer 6)

Type 3common3 practice Q

Device and protocol-to-layer matching

How to spot it:

A switch works at which layer, HTTP is a which-layer protocol, or TCP sits in which layer.

Method

Hub and repeater are layer 1; switch and bridge layer 2; router layer 3.
TCP and UDP are layer 4.
HTTP, FTP and SMTP are layer 7. IP is layer 3.
Match the job before the name.

Why it works:

A device or protocol lives at the layer of its job.

Try this

TCP and UDP work at which layer of the OSI model?

Show solution

They deliver data end to end.
They use port numbers.

Answer

Transport (layer 4)

Type 4common2 practice Q

TCP/IP model vs OSI

How to spot it:

The TCP/IP model has how many layers, which OSI layers merge into TCP/IP Application, or mapping the four TCP/IP layers.

Method

TCP/IP has 4 layers.
Network Access covers OSI 1 and 2. Internet is 3.
Transport is 4.
Application covers OSI 5, 6 and 7.

Why it works:

TCP/IP merges the top three and bottom two OSI layers.

Try this

How many layers does the TCP/IP model have?

Show solution

Network Access, Internet, Transport, Application.
That makes four.

Answer

Type 5occasional

Data unit at each layer

How to spot it:

What is the data unit at layer 2, or data is called packets at which layer.

Method

Layer 4: segments.
Layer 3: packets.
Layer 2: frames. Layer 1: bits.
Data moves down the sender and up the receiver.

Why it works:

Each layer wraps data and gives it a new name.

Try this

Data is called a frame at which OSI layer?

Show solution

Frames carry MAC addresses.
MAC belongs to Data Link.

Answer

Data Link (layer 2)

Shortcuts that save time

⚡ OSI mnemonics

Bottom-up: Please Do Not Throw Sausage Pizza Away (Physical, Data link, Network, Transport, Session, Presentation, Application). Top-down: All People Seem To Need Data Processing.

Example

Which OSI layer comes directly above the Network layer?

Show solution

Bottom-up order: Physical, Data Link, Network, Transport.
Transport follows Network.

Answer

Transport (layer 4)

⚡ Layer-device pairs

Router is layer 3. Switch and bridge are layer 2. Hub and repeater are layer 1. Use the address type: IP is 3, MAC is 2, no address is 1.

Example

At which OSI layer does a router operate?

Show solution

A router reads IP addresses.
IP belongs to the Network layer.

Answer

Layer 3 (Network)

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Counting 4 or 5 OSI layers.

OSI has 7 layers. TCP/IP has 4.

Mistake 02

Putting encryption at the Application layer.

Encryption, formatting and compression sit in Presentation (6).

Mistake 03

Saying the Session layer routes packets.

Routing is the Network layer (3). Session manages dialogues.

Mistake 04

Placing TCP and UDP at the Network layer.

TCP and UDP are Transport layer (4). IP is Network layer (3).

Quick revision

Read this the night before the exam.

OSI has 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, Application.
Router is layer 3, switch is layer 2, hub is layer 1.
Routing is Network. Encryption and compression are Presentation.
Transport layer holds TCP and UDP and uses ports. Session manages dialogues.
Data units: segments (4), packets (3), frames (2), bits (1).
TCP/IP has 4 layers: Network Access, Internet, Transport, Application.

Practice: 15 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 5 min · wrong answers go to your mistake notebook automatically.

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