Cisco CCNA OSI and TCP/IP Models 5 — Questions and Answers
Question 1: Which OSI layer is responsible for establishing, maintaining, and terminating sessions between applications?
- Transport (Layer 4)
- Presentation (Layer 6)
- Application (Layer 7)
- Session (Layer 5) (Correct answer)
Correct answer: Session (Layer 5)
The Session layer (Layer 5) controls the establishment, management, and termination of communication sessions between applications.
Question 2: In the context of the OSI model, what is a 'service data unit' (SDU)?
- The header added by each layer during encapsulation
- The data passed from a higher layer to a lower layer for encapsulation (Correct answer)
- The complete PDU including all headers and trailers
- The error-checking trailer added by Layer 2
Correct answer: The data passed from a higher layer to a lower layer for encapsulation
A Service Data Unit (SDU) is the payload (data) that a layer receives from the layer above it before adding its own header to form a PDU.
Question 3: Which of the following correctly maps OSI layers to their corresponding TCP/IP model layers?
- OSI Transport = TCP/IP Internet
- OSI Network = TCP/IP Transport
- OSI Data Link + Physical = TCP/IP Network Access (Correct answer)
- OSI Application = TCP/IP Transport
Correct answer: OSI Data Link + Physical = TCP/IP Network Access
The TCP/IP Network Access layer combines both the OSI Physical (Layer 1) and Data Link (Layer 2) layers into a single layer.
Question 4: When a host sends data using TCP, at which OSI layer is the segment header added?
- Layer 3 - Network
- Layer 5 - Session
- Layer 4 - Transport (Correct answer)
- Layer 2 - Data Link
Correct answer: Layer 4 - Transport
TCP is a Transport layer (Layer 4) protocol, and the TCP header (containing source/destination ports, sequence numbers, etc.) is added at Layer 4.
Question 5: Which statement best describes the advantage of the layered OSI model?
- It requires all vendors to use identical hardware
- It allows changes in one layer without affecting other layers (Correct answer)
- It eliminates the need for protocols at each layer
- It reduces the total number of protocols needed in networking
Correct answer: It allows changes in one layer without affecting other layers
The layered OSI model allows each layer to be updated or changed independently (e.g., switching from IPv4 to IPv6 at Layer 3) without impacting other layers.
Question 6: What is the correct order of encapsulation when a user sends an HTTP request from a web browser?
- Data → Segment → Packet → Frame → Bits (Correct answer)
- Bits → Frame → Packet → Segment → Data
- Frame → Packet → Segment → Data → Bits
- Segment → Packet → Data → Frame → Bits
Correct answer: Data → Segment → Packet → Frame → Bits
Encapsulation proceeds top-down: Application data → Transport segment → Network packet → Data Link frame → Physical bits.
Question 7: Which protocol maps IP addresses to MAC addresses and operates at the boundary between OSI Layers 2 and 3?
- ICMP
- DNS
- ARP (Correct answer)
- RARP
Correct answer: ARP
ARP (Address Resolution Protocol) resolves known IP addresses to MAC addresses, bridging the Network (Layer 3) and Data Link (Layer 2) layers.
Cisco CCNA OSI and TCP/IP Models 5
Which OSI layer is responsible for establishing, maintaining, and terminating sessions between applications?