Network Fundamentals // Lesson 01

OSI &
TCP/IP.

Networking becomes much easier when you understand what happens to data as it moves through a network. In this lesson we'll use the OSI and TCP/IP models to follow data from an application, through switches and routers, and eventually to another device.

📖 Beginner ⏱ About 15 minutes ✓ Quiz included CCNA 200-301
01 // The idea

Why do we need network models?

A network connection involves lots of different jobs. An application creates data, transport protocols manage conversations, IP provides logical addressing, Ethernet moves frames across a local network and the physical medium carries bits.

Network models divide those jobs into layers. This gives us a common language for describing what a protocol or device is doing.

Think of the layers as jobs, not boxes. Real networks don't physically contain seven separate OSI compartments. The model helps us understand which networking function is taking place.
02 // OSI model

The seven OSI layers

The OSI model has seven layers. For CCNA, you should know their order, their main functions and where common protocols and devices fit.

L7
Application
Network services used by applications. Examples include HTTP, DNS, DHCP and SMTP.
DATA
L6
Presentation
Data representation, formatting, encryption and compression.
DATA
L5
Session
Establishes, manages and terminates communication sessions.
DATA
L4
Transport
End-to-end transport using protocols such as TCP and UDP. Uses port numbers to identify applications.
SEGMENT / DATAGRAM
L3
Network
Logical addressing and routing between networks. IPv4 and IPv6 operate here.
PACKET
L2
Data Link
Local network delivery, Ethernet frames, MAC addresses and error detection.
FRAME
L1
Physical
Sends raw bits using electrical, optical or radio signals.
BITS
CCNA memory shortcut: Layer 2 thinks in MAC addresses. Layer 3 thinks in IP addresses. Layer 4 thinks in port numbers.
03 // TCP/IP

OSI vs TCP/IP

The OSI model is extremely useful for learning and troubleshooting, but the TCP/IP model more closely represents the protocol suite used by modern IP networks.

OSI

7 — Application
6 — Presentation
5 — Session
4 — Transport
3 — Network
2 — Data Link
1 — Physical

TCP/IP

Application
roughly OSI layers 5–7
Transport
roughly OSI layer 4
Internet
roughly OSI layer 3
Network Access
roughly OSI layers 1–2
04 // Interactive

Watch data become a frame

As application data moves down the networking stack, each layer adds information needed by the corresponding layer at the destination. This process is called encapsulation.

Click each stage:

HTTP DATA Application
Imagine your browser wants to send an HTTP request to a web server. At this point we're interested in the application data itself.
↓
TCP HEADER + DATA Transport
TCP adds information including source and destination port numbers. The resulting transport-layer PDU is commonly called a segment.
↓
IP HEADER + TCP SEGMENT Network
IP adds source and destination IP addresses. Routers use Layer 3 information to forward the packet towards its destination.
↓
ETHERNET HEADER + IP PACKET + TRAILER Data Link
Ethernet adds Layer 2 addressing and a trailer. The resulting frame is suitable for transmission on the local Ethernet network.
↓
01001010... Physical
The frame is represented as bits and transmitted using the physical medium — for example electrical signalling, light through fibre or radio over Wi-Fi.
05 // Real network

What happens when you open a website?

Imagine a laptop on an Ethernet LAN opening a website hosted on another network.

01 // APPLICATION

Browser

The browser generates application traffic. DNS may first be used to resolve the site's name to an IP address.

02 // SWITCH

Local LAN

Ethernet carries frames across the LAN. A switch makes forwarding decisions primarily using destination MAC addresses.

03 // ROUTER

Gateway

If the destination is on another IP network, the host sends the frame towards its default gateway. The router forwards based on the destination IP address.

04 // DESTINATION

Server

At the destination, the networking information is processed up the stack until the application receives the data.

Important: a router does not simply forward the original Ethernet frame from one network to another. It removes the incoming Layer 2 framing, processes the Layer 3 packet and creates appropriate new Layer 2 framing for the outgoing interface.
06 // Remember these

Key CCNA terms

MAC address

Layer 2 address used for local frame delivery.

IP address

Layer 3 logical address used for delivery across IP networks.

Port

Layer 4 identifier used by TCP or UDP to distinguish applications and conversations.

Frame

The Layer 2 protocol data unit.

Packet

Common name for the Layer 3 IP protocol data unit.

Encapsulation

The process of adding protocol information as data moves down the networking stack.

07 // Check your knowledge

Quick quiz

1. Which OSI layer is responsible for logical IP addressing and routing?

IPv4 and IPv6 operate at the Network layer. Routers use Layer 3 information when making IP forwarding decisions.

2. What does an Ethernet switch primarily use when making a Layer 2 forwarding decision?

A Layer 2 Ethernet switch examines the destination MAC address and uses its MAC address table to decide where to forward a frame.

3. What is the Layer 2 PDU called?

Layer 2 uses frames. Layer 3 commonly uses packets, while TCP at Layer 4 uses segments.

4. Which sequence correctly describes encapsulation as data travels down the stack?

Application data is encapsulated by the transport layer, network layer and data-link layer before being transmitted physically.

5. A router receives an Ethernet frame and needs to forward the IP packet through another Ethernet interface. What happens?

Routers process the Layer 3 packet and use appropriate Layer 2 encapsulation on each link. The Layer 2 frame is therefore not simply carried unchanged through the router.
Score: 0 / 5
Lesson complete

Next up: network devices

Next we'll look at routers, Layer 2 and Layer 3 switches, firewalls, access points, controllers, servers, endpoints and PoE — what each device actually does and where it fits into a network.