OSI Model (Open Systems Interconnection)

A conceptual framework that standardizes how different network systems communicate. It breaks communication into 7 layers, each with a specific job.

The 7 Layers (Top → Bottom)

#	Layer	What it does	Examples
7	Application	Interface for the end user/app	HTTP, FTP, DNS, SMTP
6	Presentation	Translation, encryption, compression	SSL/TLS, JPEG, ASCII
5	Session	Opens, manages, closes sessions	NetBIOS, RPC
4	Transport	End-to-end delivery, reliability	TCP, UDP
3	Network	Logical addressing, routing	IP, ICMP, routers
2	Data Link	Physical addressing, error detection	MAC, Ethernet, switches
1	Physical	Raw bits over the wire	Cables, hubs, signals

Layer-by-Layer Detail

Layer 7 – Application

• Closest to the user. Protocols: HTTP, DNS, FTP.
• It is NOT the app itself (like Chrome) — it is the protocol the app uses.

Layer 6 – Presentation

• Translates data into a format the app can understand.
• Handles encryption (TLS), compression, and encoding (UTF-8, ASCII).
• Often considered the least discussed layer in interviews.

Layer 5 – Session

• Manages sessions — establishing, maintaining, and terminating connections.
• Think of it as managing a "conversation" between two devices.

Layer 4 – Transport ★ (very interview-heavy)

• Responsible for end-to-end communication.
• TCP → reliable, ordered, connection-oriented (uses handshakes).
• UDP → fast, connectionless, no guarantee of delivery.
• Introduces ports (e.g. HTTP = 80, HTTPS = 443).
• Handles segmentation — breaking large data into smaller chunks.

Layer 3 – Network ★

• Deals with logical addressing (IP addresses) and routing.
• Routers operate here.
• Decides the best path for data across networks.

Layer 2 – Data Link ★

• Deals with physical (MAC) addresses.
• Responsible for node-to-node delivery within the same network.
• Switches operate here.
• Detects (sometimes corrects) errors from Layer 1.

Layer 1 – Physical

• Raw transmission of bits (0s and 1s) over a physical medium.
• Cables, fiber optics, radio signals, hubs.

How Data Flows — Encapsulation & Decapsulation

When you send data, each layer wraps it with its own header (called encapsulation). On the receiving end, each layer unwraps it (decapsulation).

Sender (goes DOWN — Encapsulation):

Layer 7 - Application   → [Data]
Layer 6 - Presentation  → [Data]
Layer 5 - Session       → [Data]
Layer 4 - Transport     → [L4 Header | Data]              → Segment
Layer 3 - Network       → [L3 | L4 | Data]                → Packet
Layer 2 - Data Link     → [L2 | L3 | L4 | Data]            → Frame
Layer 1 - Physical      → 101010...                        → Bits on wire

Receiver (goes UP — Decapsulation):

Layer 1 - Physical      → receives raw bits
Layer 2 - Data Link     → strips L2 header, checks MAC
Layer 3 - Network       → strips L3 header, checks IP
Layer 4 - Transport     → strips L4, checks port, reassembles
Layer 5 - Session       → manages session
Layer 6 - Presentation  → decrypts/decompresses
Layer 7 - Application   → hands data to the app ✓

Devices in the Middle

Device	Layers it processes
Hub	L1 only — just forwards bits
Switch	L1 + L2 — reads MAC, forwards frame
Router	L1 + L2 + L3 — reads IP, routes packet
Firewall / Load Balancer	Up to L4 or L7 depending on type

Router example:

Incoming: Bits → Frame → Packet → [reads IP] → Packet → Frame → Bits : Outgoing
                          UP to L3 ↑         DOWN from L3 ↓

Mental Model

• Sending = going DOWN (wrapping/encapsulating)
• Receiving = going UP (unwrapping/decapsulating)
• Middle devices = go up only as far as they need, then back down

Common Interview Questions

Q: Why does the OSI model matter? It gives a common language to troubleshoot and design networks. When something breaks, you can isolate which layer the problem is at.

Q: A website isn't loading — which layer do you check? Go bottom-up: Is the cable connected? (L1) → Is there a network? (L2/L3) → Can you ping? (L3) → Can you reach the port? (L4) → Is the app responding? (L7)

Mnemonic (top to bottom): All People Seem To Need Data Processing