Networking: Devices, Topologies & Protocols

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OSI and TCP/IP reference models

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

The idea in one minute

OSI (Open Systems Interconnection) - 7 layers, top-down:

#	Layer	Function	Works with
7	Application	User-facing network services	HTTP, FTP, SMTP, DNS
6	Presentation	Format, encryption, compression	JPEG, MP3, SSL/TLS concepts
5	Session	Opens/manages/closes dialogues	NetBIOS, RPC
4	Transport	End-to-end delivery, ports, reliability	TCP, UDP (data units: segments)
3	Network	Logical addressing, routing	IP, routers (packets)
2	Data Link	Framing, MAC addressing, error detect	Ethernet, switches/bridges (frames)
1	Physical	Raw bits on the medium	Cables, hubs, repeaters

TCP/IP model compresses these into 4: Application (7+6+5), Transport (4), Internet (3), Network Access/Link (2+1) - it is what the real Internet runs.

Encapsulation journey: data -> segments (L4) -> packets (L3) -> frames (L2) -> bits (L1).

Why a reference model exists

Moving data from one machine to another needs many different jobs: signalling, framing, addressing, delivery, dialogue control, translation, and the user's service. The OSI (Open Systems Interconnection) model splits these into 7 layers, so each layer can be designed and replaced independently.

The 7 layers — bottom to top

#	Layer	One-line job	Works with
1	Physical	raw bits on cables/signals	cables, hubs, repeaters
2	Data Link	frames, MAC addresses, error detection	switches, bridges, NIC
3	Network	IP addressing and routing	routers, IP
4	Transport	end-to-end delivery, segmentation, ports	TCP, UDP
5	Session	opens, manages, closes dialogues	session setup tokens
6	Presentation	translation, encryption, compression	formats, SSL/TLS
7	Application	the user-facing services	HTTP, FTP, SMTP, browsers

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

How the layers are tested

Device-to-layer: hub/repeater = 1, switch/bridge = 2, router = 3.
Function-to-layer: routing = Network (3); framing/MAC = Data Link (2); encryption and compression = Presentation (6); mail/web services = Application (7).
Number questions: 7 layers (OSI) vs 4 layers (TCP/IP). The middle is Transport in both.
Data flows DOWN the sender's stack (7 to 1), across the wire, and UP the receiver's stack (1 to 7).

The TCP/IP model — the practical cousin

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

The Internet actually runs on TCP/IP; OSI is the teaching map. "Which model has 4 layers?" — TCP/IP.

Reading a layer question in the exam

First decide what the question is really asking: a number (7 layers, 4 in TCP/IP), an order (which sits above which), a function (what does this layer do), or a worker (which device or protocol lives here). For function questions, test the planted option against its neighbour: routing is NOT Transport (Transport only delivers end to end after Network has found the path), and encryption is NOT Application (Application serves the user; Presentation prepares the data format for it).

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?', correct ordering questions, or the layer that sits directly above/below another.

Method

OSI = 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, Application (bottom to top).
Mnemonic top-down: All People Seem To Need Data Processing.
Transport is the middle (4th); Application is 7th, Physical 1st.

Try this

How many layers are there in the OSI reference model?

Show solution

7 — the TCP/IP model has only 4; the count mix-up is the standard trap.

Type 2very common4 practice Q

Function-to-layer matching

How to spot it:

'Routing is done at which layer?', 'encryption/compression is a ___ layer function', 'framing and MAC addressing belong to ___'.

Method

Bits/cables = Physical (1); frames, MAC = Data Link (2); addressing and routing = Network (3); end-to-end delivery, ports = Transport (4); dialogues = Session (5); translation, encryption, compression = Presentation (6); user services = Application (7).
The planted traps: encryption at Application, routing at Transport.
Ports and TCP/UDP live in Transport, never Network.

Try this

Encryption and data compression are functions of which OSI layer?

Show solution

Presentation (Layer 6) — it prepares data formats before the Application's services use them.

Type 3common3 practice Q

Device/protocol-to-layer matching

How to spot it:

'A switch works at which layer', 'HTTP is a ___ layer protocol', 'TCP sits in which layer'.

Method

Hub/repeater L1; switch/bridge L2; router L3; TCP/UDP L4.
HTTP, FTP, SMTP = Application (7); IP = Network (3).
Match the job before the name: a router routes (3), a switch switches frames (2).

Try this

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

Show solution

Transport (Layer 4) — end-to-end delivery, segmentation and port numbers.

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 = 4 layers: Network Access (1+2), Internet (3), Transport (4), Application (5+6+7).
OSI's top three merge into TCP/IP's Application.
The Internet runs on TCP/IP; OSI is the reference map.

Try this

How many layers does the TCP/IP model have?

Show solution

4 — Network Access, Internet, Transport and Application (the last swallowing OSI layers 5-7).

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

Transport (P-D-N-T: layer 4).

⚡ Layer-device pairs

Router = layer 3 (both R words rhyme with 'routing/IP'); Switch/Bridge = layer 2 (frames/MAC); Hub/Repeater = layer 1 (bits). Any 'works at which layer' question resolves by the device's address type: IP = 3, MAC = 2, no address = 1.

Example

At which OSI layer does a router operate?

Show solution

Layer 3 - Network.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Counting 4 or 5 OSI layers - the model has 7 (the TCP/IP model has 4).

Mistake 02

Putting encryption at the Application layer - classic encryption/formatting sits in Presentation (6).

Mistake 03

Saying the Session layer routes packets - routing is Network (3).

Quick revision

Read this the night before the exam.

OSI = 7 layers: Physical, Data Link, Network, Transport, Session, Presentation, Application.
Router L3, Switch L2, Hub L1; routing = Network; encryption/compression = Presentation.
Transport layer = TCP/UDP and ports; Session = dialogues.
TCP/IP model = 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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