BABassel AasiCloud & DevOps Engineer
Training / Certifications / CompTIA Network+ / The OSI Model and Encapsulation

CompTIA Network+ (N10-009) · Lesson 1

The OSI Model and Encapsulation

Name the seven OSI layers in order, explain what each layer does and which units, addresses and devices belong to it, and describe encapsulation and decapsulation.

Domain 1.0: Networking Concepts (23%)Objective 1.1About 5:22CaptionsStatic view for reduced motion
Lesson 1: The OSI model and encapsulation Illustrative example: addresses are samples CompTIANetwork+N10-009Up to 90 questionsmultiple choice and performance-based90 minutesPassing score: 720on a 100–900 scale5 domainsDomain 1.0: Networking Concepts23%Domain 2.0: Network Implementation20%Domain 3.0: Network Operations19%Domain 4.0: Network Security14%Domain 5.0: Network Troubleshooting24%This lesson: Domain 1.0, objective 1.1 (the OSI reference model).1PhysicalLayer 12Data linkLayer 23NetworkLayer 34TransportLayer 45SessionLayer 56PresentationLayer 67ApplicationLayer 7Two reasons it mattersA common language“Our switch worksat Layer 2” means thesame thing to everyone,whoever made itA troubleshooting orderCheck the cable first,then addresses, thenports and applications:bottom upLayer 1Layer 77 layers, each with one job,serving the layer aboveBottom up: Please Do Not Throw Sausage Pizza AwayPlease→ P for PhysicalDo→ D for Data linkNot→ N for NetworkThrow→ T for TransportSausage→ S for SessionPizza→ P for PresentationAway→ A for ApplicationTop down works too: All People Seem To Need Data Processing.Layer 1: PhysicalUnit: bitsSends bits as electrical, light or radio signalsCables, connectors, pins and signal timingDevices here repeat signals without reading them1 0 1 1 0 0 1 0 1 1 0 1Copper cableFiber opticRadio (Wi-Fi)Devices: hubs, repeaters, media convertersLayer 2: Data linkUnit: framesDelivers frames on the same local networkUses MAC addresses (network card hardware IDs)Checks each frame for errors on arrivalA frame (simplified)Dest. MACSource MACPayload (the packet)FCSFCS: frame check sequence, used to detect errorsSublayers: media access control (MAC) and logical link control (LLC)Devices: switches, wireless access pointsLayer 3: NetworkUnit: packetsMoves packets between different networksUses logical addresses: IP addressesRouters choose the path (routing)Office laptop192.168.1.20RouterInternetStore's server203.0.113.10Layer 4: TransportUnits: segments, datagramsCarries data from one application to anotherPort numbers name the application (443 = HTTPS)TCP for reliability, UDP for speedTCP: connection-orientedSYN → SYN-ACK → ACKOrdered, acknowledged,lost data resentUnit: segmentUDP: connectionlessNo handshakeLess overhead, no deliveryguarantee: voice, video, DNSUnit: datagramLayers 5, 6 and 7: closest to the user7ApplicationNetwork services thatprograms use:HTTP, DNS, SMTP6PresentationFormats data so bothsides understand it;encrypts, compresses5SessionSets up, managesand ends sessionsbetween applicationsCareful: the browser itself isn't Layer 7.Layer 7 is the protocols it uses, like HTTP and DNS.Sending: each layer adds its own headerDataDataTCP headerDataSegmentIP headerTCP headerDataPacketMAC headerIP headerTCP headerDataFCSFrame↓↓1 0 1 1 0 0 1 0 1 1 0 1 0 0 1 1BitsSending host: encapsulation, from Layer 7 down to Layer 1.Receiving host: decapsulation, from Layer 1 up, removing each header in turn.The FCS is a trailer: Layer 2 adds it at the end of the frame.If the question mentions……think“MAC address, switch, frame”Layer 2: Data link“IP address, router, packet”Layer 3: Network“Port number, TCP or UDP”Layer 4: Transport“Cable, connector, hub, bits”Layer 1: Physical“Encryption, compression, data format”Layer 6: Presentation“HTTP, DNS, SMTP”Layer 7: ApplicationUnit (PDU)AddressExamplesData–HTTP, DNS, SMTPData–Encryption, formatsData–SessionsSegment / datagramPortTCP, UDPPacketIP addressRouterFrameMAC addressSwitch, access pointBits–Cable, hub, repeater

Tip: turn your phone sideways or use full screen for larger labels.

Captions match the narration word for word. Press Space to play or pause and the arrow keys to change scenes.

Learning objective

Name the seven OSI layers in order, explain what each layer does and which units, addresses and devices belong to it, and describe encapsulation and decapsulation.

Exam mapping

Domain 1.0: Networking Concepts (23%). Objective 1.1 in the official N10-009 exam objectives.

Key takeaways

  • The seven OSI layers, from the bottom: 1 Physical, 2 Data link, 3 Network, 4 Transport, 5 Session, 6 Presentation, 7 Application.
  • Layer 1 moves bits as signals (cables, connectors, hubs, repeaters). Layer 2 delivers frames on the local network using MAC addresses (switches, access points).
  • Layer 3 routes packets between networks using IP addresses (routers). Layer 4 uses port numbers; TCP is reliable and connection-oriented, UDP is connectionless.
  • Layer 5 manages sessions, Layer 6 formats, encrypts and compresses data, and Layer 7 provides network services such as HTTP and DNS.
  • Encapsulation adds a header at each layer on the way down (segment, packet, frame, then bits); decapsulation removes them on the way up.

Exam tips

How the exam asks about this

Questions describe a situation and expect you to recognize the concept behind it.

If the question mentions…Think
A MAC address, a switch or a frameLayer 2: Data link
An IP address, a router or a packetLayer 3: Network
A port number, TCP or UDP, a segmentLayer 4: Transport
A cable, connector, hub, repeater or bitsLayer 1: Physical
Encryption, compression or data formattingLayer 6: Presentation
HTTP, DNS, SMTP and other application protocolsLayer 7: Application

Common traps

  • Count from the bottom: Layer 1 is Physical, and Layer 7 is Application.
  • A web browser or email program isn't the Layer 7 application in OSI terms; Layer 7 is the protocols and services it uses, such as HTTP and SMTP.
  • Hubs and repeaters are Layer 1: they repeat signals out of every port without reading addresses. Switches read MAC addresses (Layer 2), and routers read IP addresses (Layer 3).
  • Know the units: bits (Layer 1), frames (Layer 2), packets (Layer 3), and segments for TCP or datagrams for UDP (Layer 4).

Practice

Exam-style questions

Answer each one before you open the explanation.

Question 1. A user can't connect to the office network. A technician finds that the connector on the user's patch cable is cracked. At which OSI layer is the problem?

  1. Layer 1: Physical
  2. Layer 2: Data link
  3. Layer 3: Network
  4. Layer 7: Application
Show answer

A. Layer 1: Physical. Cables and connectors are Layer 1. The other layers can't work until the physical connection does, which is why troubleshooting usually starts at the bottom.

Question 2. Which device forwards traffic based on the destination MAC address in each frame?

  1. Hub
  2. Router
  3. Switch
  4. Media converter
Show answer

C. Switch. A switch is a Layer 2 device that reads MAC addresses. A hub and a media converter are Layer 1 devices that don't read addresses, and a router forwards packets by IP address at Layer 3.

Question 3. What is the protocol data unit (PDU) at the network layer of the OSI model?

  1. Frame
  2. Packet
  3. Segment
  4. Bit
Show answer

B. Packet. Layer 3 works with packets. Frames are Layer 2, segments are Layer 4 (TCP), and bits are Layer 1.

Question 4. A live voice application needs low delay and can tolerate a little data loss. Which protocol and OSI layer fit best?

  1. TCP at Layer 3
  2. IP at Layer 4
  3. TCP at Layer 4
  4. UDP at Layer 4
Show answer

D. UDP at Layer 4. UDP is a connectionless Layer 4 protocol with less overhead, so it suits real-time traffic. TCP is also Layer 4, but its acknowledgments and resending add delay. IP is a Layer 3 protocol.

Question 5. A host is sending data across the network. Which TWO things happen during encapsulation? (Select TWO.)

  1. The data link layer removes the frame header
  2. The transport layer adds port numbers to the data
  3. The application layer adds MAC addresses
  4. The physical layer adds IP addresses
  5. The network layer adds source and destination IP addresses
Show answer

B. The transport layer adds port numbers to the data and E. The network layer adds source and destination IP addresses. On the way down, Layer 4 adds a header with port numbers and Layer 3 adds a header with IP addresses. Removing headers is decapsulation on the receiving host, MAC addresses are added at Layer 2, and IP addresses at Layer 3.

Transcript

Lesson transcript

The full narration, scene by scene. The captions file contains the same text with timings.

Scene 1 · 0:00

Lesson 1: The OSI model and encapsulation. Welcome to the first lesson of this free CompTIA Network+ course. The N10-009 exam has five domains, and we start with Domain 1, Networking Concepts, which is 23 percent of the exam. Objective 1.1 asks you to explain the OSI reference model. By the end of this lesson, you'll know all seven layers, what each one does, and how data is wrapped and unwrapped as it crosses a network.

Scene 2 · 0:30

Why a reference model?. The OSI model, short for Open Systems Interconnection, describes networking as seven layers stacked on top of each other. Each layer has one job and serves the layer above it. That gives everyone a common language: a vendor can say its switch works at Layer 2, and every network engineer knows what that means. It also gives you a troubleshooting order: start at the bottom and work up.

Scene 3 · 1:00

Seven layers, numbered from the bottom. Layers are numbered from the bottom. Layer 1 is Physical, then Data link, Network, Transport, Session, Presentation, and Layer 7, Application, at the top. A common way to remember them from the bottom up is: Please Do Not Throw Sausage Pizza Away. The exam uses both the numbers and the names, so learn them together, in both directions.

Scene 4 · 1:25

Layer 1: Physical. Layer 1, the Physical layer, moves raw bits: ones and zeros, sent as electrical signals on copper, pulses of light in fiber, or radio waves for Wi-Fi. It covers cables, connectors, pins and signal timing. Layer 1 devices pass signals along without reading them: hubs, repeaters and media converters. In a small office, a damaged patch cable at someone's desk is a Layer 1 problem.

Scene 5 · 1:54

Layer 2: Data link. Layer 2, the Data link layer, delivers frames between devices on the same local network. It uses MAC addresses, the hardware addresses of network interface cards, and it checks each frame for errors when it arrives. It has two sublayers: media access control and logical link control. Switches work here: a switch reads the destination MAC address and forwards the frame only out of the right port.

Scene 6 · 2:23

Layer 3: Network. Layer 3, the Network layer, moves packets between different networks. It uses logical addresses, called IP addresses, and it picks a path for each packet, which is called routing. Routers work at Layer 3, and so do multilayer switches. When the office laptop opens an online store's website, routers forward its packets from the office network, across the internet, to the store's server.

Scene 7 · 2:50

Layer 4: Transport. Layer 4, the Transport layer, carries data from one application to another. Port numbers identify the application, like 443 for HTTPS. TCP is connection-oriented: it sets up a connection with a three-way handshake, keeps data in order, and resends anything lost. Its unit is the segment. UDP is connectionless: it sends datagrams with less overhead and no delivery guarantee, which suits voice, video and DNS lookups.

Scene 8 · 3:19

Layers 5 to 7: closest to the user. The top three layers are closest to the user. Layer 5, Session, sets up, manages and ends the conversations between applications. Layer 6, Presentation, formats data so both sides understand it, and can encrypt and compress it. Layer 7, Application, provides the network services that programs use, such as HTTP, DNS and email protocols. Careful: the web browser itself isn't Layer 7. The protocols it uses are.

Scene 9 · 3:48

Encapsulation: wrapping data on the way down. Now let's follow the data down the stack. The browser's request starts as data. Layer 4 adds a TCP header with port numbers, which makes a segment. Layer 3 adds an IP header with IP addresses, which makes a packet. Layer 2 adds a header with MAC addresses and a trailer for error checking, which makes a frame. Layer 1 sends it as bits. The server reverses all of this. That's called decapsulation.

Scene 10 · 4:20

How the exam asks about this. On the exam, map each clue to its layer. MAC addresses, switches and frames mean Layer 2. IP addresses, routers and packets mean Layer 3. Port numbers, TCP and UDP mean Layer 4. Cables, hubs and bits mean Layer 1. Encryption and data formatting point to Layer 6, and protocols such as HTTP and DNS point to Layer 7. Expect scenario questions that ask at which layer a device or a problem sits.

Scene 11 · 4:52

Recap: the OSI model. To recap: the OSI model has seven layers, from Physical at the bottom to Application at the top. Bits, frames, packets and segments are the units at Layers 1 to 4, with MAC addresses at Layer 2, IP addresses at Layer 3 and port numbers at Layer 4. Sending wraps data in headers, and receiving unwraps it. Next, we'll look at the networking appliances, applications and functions in objective 1.2.

Download captions (WebVTT)

Sources and accuracy

Checked against the official CompTIA N10-009 exam objectives and the standards below on October 24, 2026. Diagrams and addresses in the animation are illustrative.

This free course is independent and isn't affiliated with or endorsed by CompTIA. CompTIA and Network+ are trademarks of CompTIA, Inc.

← Course overviewCompTIA Network+ (N10-009)
Lesson 2Coming soon

Preparing a team for Network+?

I run hands-on networking and cloud training for teams, built around your own environment and goals.

Book a consultation Email me