1.2 OSI Model Concepts Review Cybersecurity Associate

At which OSI layer does a router mainly operate when making decisions about where to forward packets?

• Layer 1

• Layer 2

• Layer 3

• Layer 4

Answer explanation

Explanation: Routers operate at the Network layer (Layer 3), where they use IP addresses to determine the best path for data packets to travel.
Real-World Scenario: When you browse a website hosted in another country, your router communicates with other routers using Layer 3 IP addresses to ensure your data reaches the correct server across the internet.

A network admin finds that a client computer is misidentifying data packets, treating an email packet as if it were web traffic. This is most likely a problem at which OSI process?

• The client computer isn’t receiving any packets.

• The client receives an email packet but interprets it as web traffic

• The client processes VoIP packets correctly.

• The client cannot connect to port 80.

• The client is only experiencing slowness from congestion.

Answer explanation

Explanation: This is a Transport layer (Layer 4) de-multiplexing issue. The port numbers that separate services (email vs. web) are being misinterpreted.
Real-World Scenario: If your laptop mistakes Outlook email traffic for a web browser request, it’s because Layer 4 port assignment has gone wrong, similar to mailing a letter to the wrong department at the same office.

Which of the following devices and security tools operate at OSI Layer 4 to strengthen network defenses?

• Intrusion Detection Systems (IDS) that analyze traffic for suspicious behavior

• Antivirus software on a host computer

• Firewalls performing stateful packet inspection

• Routers filtering IP packets

• Switches dividing collision domains

Answer explanation

Explanation: Both IDS and stateful firewalls analyze traffic at Layer 4 (Transport), using TCP/UDP ports and session data to determine suspicious or unauthorized activity.
Real-World Scenario: A firewall might block unusual attempts to connect to TCP port 23 (Telnet), while IDS monitors and alerts when repeated suspicious login attempts happen.

Which three are valid layers in the OSI reference model?

• Presentation Layer

• Transmission Layer

• Session Layer

• Application Layer

• WAN Layer

Answer explanation

Explanation: The OSI model has 7 defined layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. The distractors (WAN, Encryption, etc.) are not official layers.
Real-World Scenario: When you stream Netflix, the Presentation layer handles video format, the Session layer keeps your stream stable, and the Application layer delivers the app itself.

At which OSI layer does physical addressing (MAC addresses) occur?

• Physical layer

• Data Link layer

• Session layer

• Application layer

Answer explanation

Explanation: The Data Link layer (Layer 2) is where physical addressing takes place, using MAC addresses burned into NICs to identify devices.
Real-World Scenario: When two laptops connect over Wi-Fi, their NICs use unique MAC addresses to communicate within the same local network.

Which organization developed the OSI reference model?

• ITU

• IETF

• IEEE

• ISO

Answer explanation

Explanation: The OSI model was created by ISO (International Organization for Standardization) to provide a universal framework for network communication.
Real-World Scenario: Just like ISO sets standards for screw sizes so hardware fits globally, it also standardized OSI so networks could interoperate worldwide.

Which OSI layer is responsible for defining the physical network medium, such as cables and signals?

• Layer 5
• Layer 1

• Layer 3

• Layer 7

Answer explanation

Explanation: The Physical layer (Layer 1) deals with tangible elements like cabling, connectors, and signaling methods.
Real-World Scenario: If an Ethernet cable is cut, it’s a Layer 1 failure — the problem is with the physical medium itself.

Which OSI layer manages the start, maintenance, and termination of a dialogue between two applications?

• Session Layer

• Responsible for routing packets

• Converts between ASCII and Unicode

• Physical transmission of data

Answer explanation

Explanation: The Session layer (Layer 5) ensures sessions are created, maintained, and closed properly.
Real-World Scenario: A Zoom call relies on the Session layer to keep the meeting active and then close it cleanly when you hang up.

What is the primary function of encapsulation in networking?

• Convert data into analog

• Encrypt data

• Compress data

• Add headers for routing and delivery

Answer explanation

Explanation: Encapsulation adds headers (and sometimes trailers) at each OSI layer, providing the instructions needed for delivery.
Real-World Scenario: Like putting a shipping label and return address on a package, encapsulation adds control info so the network knows where to send the data.

Which three fields are typically added by the Data Link layer during encapsulation?

• Window size

• Checksum for error checking

• Source hardware address

• Destination hardware address

• Sequence number

Answer explanation

Explanation: The Data Link layer frame includes the source and destination MAC addresses and a checksum (Frame Check Sequence) to verify integrity.
Real-World Scenario: When you send a print job over Wi-Fi, your computer’s NIC adds its MAC address, the printer’s MAC address, and a checksum so the printer knows the data is valid.