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OSI model practice questions with answers

Updated October 9, 2026

Fifteen free OSI model practice questions with explained answers, written as short scenarios rather than memory drills. The OSI model splits network communication into seven layers, and the exam skill is placing a symptom, a device or a protocol data unit on the correct layer. The material is tested in CompTIA Network+ (N10-009), where objective 1.1 covers the layers directly.

How to use these questions

Answer each question before opening the explanation, and commit to one letter. Underline the constraint word in the stem (for example “first”, “only”, “most likely”), because it usually separates the right option from a plausible one. Wrong options are explained as well, so read the whole explanation even after a correct answer. The OSI vs TCP/IP guide covers the theory behind every question.

Warm-up: layers and data units

Question 1. A router examines the destination IP address of an incoming packet to decide which interface to forward it out of. At which OSI layer is this decision made?

  • A. Layer 2, Data link
  • B. Layer 3, Network
  • C. Layer 4, Transport
  • D. Layer 5, Session
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Answer: B

Logical addressing and path selection between networks belong to Layer 3, the Network layer. Layer 2 (Data link) uses MAC addresses within one local segment. Layer 4 (Transport) deals with ports and end-to-end delivery. Layer 5 (Session) manages dialogues between applications and plays no role in forwarding.

Question 2. A host sends a file over TCP. What is the name of the protocol data unit created at the Transport layer for this transfer?

  • A. Frame
  • B. Packet
  • C. Bit
  • D. Segment
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Answer: D

TCP data at Layer 4 is called a segment (a UDP unit is a datagram). A frame is the Layer 2 unit, a packet is the Layer 3 unit, and a bit is the Layer 1 unit.

Question 3. Under the OSI model, which layer is responsible for translating data formats, such as character encoding, compression and encryption, so that the receiving application can read it?

  • A. Presentation
  • B. Session
  • C. Application
  • D. Transport
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Answer: A

The Presentation layer (Layer 6) handles syntax, encoding, compression and encryption. The Session layer (5) sets up and tears down dialogues. The Application layer (7) is the interface to network services for the user program. The Transport layer (4) provides end-to-end delivery and does not translate formats.

Question 4. The link light on a workstation network card is off and the patch cable has a bent connector. Following a bottom-up approach, at which layer does this fault sit?

  • A. Data link
  • B. Network
  • C. Physical
  • D. Transport
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Answer: C

A damaged cable or connector is a Layer 1 (Physical) fault, since this layer covers media, signalling and connectors. Data link faults involve framing, MAC addressing or switch port state. Network layer faults involve IP addressing or routing. Transport faults involve ports or TCP behaviour; none of those explain a dark link light.

Question 5. In the four-layer TCP/IP model, which layer corresponds to the OSI Session, Presentation and Application layers combined?

  • A. Internet
  • B. Application
  • C. Link
  • D. Transport
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Answer: B

The TCP/IP Application layer covers OSI layers 5 to 7. The Internet layer maps to OSI Network, the Transport layer maps to OSI Transport, and the Link (Network access) layer maps to OSI Physical plus Data link.

Scenarios: placing symptoms and devices

Question 6. A technician verifies layers 1 to 3 for a user: the link is up, the IP configuration is correct and ping to the web server by IP address succeeds. The browser still reports that the site name cannot be found. At which layer does the problem most likely sit?

  • A. Application
  • B. Network
  • C. Data link
  • D. Physical
Show answer

Answer: A

Name resolution is performed by DNS, an Application layer protocol, and layers 1 to 3 are already proven good. A Network layer fault would also break the ping by IP address. A Data link or Physical fault would break the ping as well.

Question 7. A packet travels from a client through two routers to a server, with no NAT in the path. Which header fields are rewritten at every router hop?

  • A. Source and destination IP addresses
  • B. Destination port number
  • C. Source and destination MAC addresses
  • D. Protocol number
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Answer: C

Each router strips the incoming Ethernet header and builds a new one, so the source and destination MAC addresses change on every hop. The source and destination IP addresses stay the same end to end when there is no NAT. The destination port and the protocol number are untouched by routing.

Question 8. Which sequence shows the correct order of encapsulation on a sending host, starting with the application data?

  • A. Data, frame, packet, segment, bits
  • B. Data, packet, segment, frame, bits
  • C. Data, segment, frame, packet, bits
  • D. Data, segment, packet, frame, bits
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Answer: D

Data is wrapped as a segment at Layer 4, then a packet at Layer 3, then a frame at Layer 2, and finally sent as bits at Layer 1. The other sequences swap the packet, segment or frame steps, which contradicts the downward order of the layers.

Question 9. A protocol must establish a dialogue between two applications, maintain it, set checkpoints so an interrupted transfer can resume, and close it cleanly. Which OSI layer provides these functions?

  • A. Transport
  • B. Session
  • C. Presentation
  • D. Data link
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Answer: B

Dialogue control, checkpointing and orderly termination are Session layer (Layer 5) functions. The Transport layer provides segmentation, reliability and ports. The Presentation layer handles data format and encryption. The Data link layer works only between adjacent nodes on one link.

Question 10. A legacy network uses a hub to connect eight workstations. At which OSI layer does the hub operate?

  • A. Layer 1, Physical
  • B. Layer 2, Data link
  • C. Layer 3, Network
  • D. Layer 4, Transport
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Answer: A

A hub regenerates and repeats the electrical signal out of all other ports without reading addresses, which is a Layer 1 (Physical) function. A switch operates at Layer 2 because it reads MAC addresses. A router operates at Layer 3. A load balancer typically works at Layer 4 or above.

Edge cases and traps: mappings and proof of function

Question 11. Which statement about the TCP/IP Internet layer and the OSI model is correct?

  • A. It maps to OSI layers 1 and 2
  • B. It maps to OSI layers 2 and 3
  • C. It maps to OSI layers 4 and 5
  • D. It maps to OSI layer 3 only
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Answer: D

The TCP/IP Internet layer (IP, ICMP) maps to the OSI Network layer, which is Layer 3 only. It does not include Layer 2 (that belongs to the Link layer), and it does not include the Transport layer or the Session layer.

Question 12. A packet capture shows a TCP three-way handshake between a client and a web server completing successfully, yet the client then receives an HTTP 503 error. What does the completed handshake prove?

  • A. Only that Layer 4 works
  • B. Only that layers 5 to 7 work
  • C. That layers 1 to 4 work
  • D. That only Layer 7 works
Show answer

Answer: C

A completed TCP handshake proves that layers 1 to 4 work end to end between the two hosts: the path, addressing, routing and the Transport layer. The 503 response is generated by the application, so the fault lies in layers 5 to 7 on the server side. The handshake does not prove the application is healthy, and it proves much more than Layer 4 alone.

Question 13. A wireless access point receives 802.11 frames from clients and forwards the traffic to the wired LAN as 802.3 Ethernet frames. Which OSI layer is mainly responsible for this conversion?

  • A. Data link
  • B. Physical
  • C. Network
  • D. Transport
Show answer

Answer: A

Converting between 802.11 and 802.3 frame formats is a Data link (Layer 2) function, because both are Layer 2 framing standards. The Physical layer handles the radio and cable signalling but not the frame format. The Network layer would route between IP subnets, which an access point acting as a bridge does not do. The Transport layer is not involved in frame conversion.

Question 14. A router forwards a packet from one Ethernet interface to another Ethernet interface toward the next hop. What happens to the Layer 2 header and trailer of the incoming frame?

  • A. It is copied unchanged to the outgoing frame
  • B. It is encrypted before forwarding
  • C. It is removed and replaced with a new one
  • D. A second header is appended after it
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Answer: C

The router removes the incoming frame header and trailer (de-encapsulation) and builds a new frame with the MAC addresses for the next hop. Copying the old header unchanged would carry the wrong MAC addresses. Layer 2 headers are not encrypted by routing. Appending a second header after the existing one is not how encapsulation works for a forwarded packet.

Question 15. Which pairing of OSI layer, protocol data unit and typical device is correct?

  • A. Layer 4, frame, switch
  • B. Layer 3, frame, router
  • C. Layer 1, packet, hub
  • D. Layer 2, frame, switch
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Answer: D

Layer 2 uses frames and a switch forwards them by MAC address. A hub is Layer 1 and works with bits, not packets. A router is Layer 3 and forwards packets, not frames. A switch forwarding segments at Layer 4 mixes up the unit and the layer.

Where to go next

Read the full OSI vs TCP/IP guide for the layer-by-layer reference. Related practice sets: network port practice questions and VLAN and trunking practice questions. For a timed run, take the free 20-question Network+ practice test, and review the multiple-choice exam tips before exam day.