Top Questions and Answers on OSI Model

by | Sep 19, 2016 | CCNA | 10 comments

List the layers of OSI?

From top to bottom, OSI layers are,

Application, Presentation, Session, Transport, Network, Data Link, and Physical.

What are the responsibilities of the Data Link Layer?

Framing, Error detection, CRC, and Physical Addressing is the task of DLL.

What are the responsibilities of the Network Layer?

Routing, IP Addressing, and Path determination are the main responsibilities of the Network Layer.

What are the responsibilities of the Transport Layer?

Transport Layer has a lot of functions. Most important being,

  • Multiplexing and De-Multiplexing
  • Segmentation and Re-assembly
  • Flow Control
  • Error Correction
  • Connection Establishment
  • Sequencing
  • Acknowledgment
  • 3-way Handshake

Routers work at which OSI layer?

Network Layer

Switches work at which OSI layer?

Layer 2 and Some Switches can operate at Layer 3 and above

What is a Window in networking terms?

The window is the number of segments sent by TCP between two acknowledgments.

What is the role of the LLC sublayer in the data link layer?

Logical Link Control provides error detection, using Ethernet trailer field frame check sequence (FCS).

What is the function of the Application Layer in networking?

Application Layer is responsible for providing a user interface between user and Network with the help of applications like web browsers.

What is the difference between TCP and UDP?

Following are differences in TCP and UDP,

  • TCP stands for “Transmission Control Protocol” UDP stands for “User Datagram Protocol”.
  • TCP is a connection-oriented protocol while UDP is a connectionless protocol.
  • TCP is more reliable than UDP.
  • UDP is faster for data sending than TCP.
  • UDP makes error checking but no reporting but TCP checks for errors and performs reporting.
  • TCP provides guaranteed Delivery of Data but UDP has no guarantee.
  • The header size of TCP is 20 bytes while that of UDP is 8 bytes.
  • TCP has acknowledgment segments but UDP has no acknowledgment.
  • TCP is used for applications that require high reliability but less time-critical whereas UDP is used for applications that are time-sensitive but require less reliability.

What is the port no of DNS and Telnet?

DNS = 53, Telnet = 23

Which service uses both TCP and UDP?

DNS uses both TCP and UDP

What is the port no of SMTP and POP3?

POP3 = 110; SMTP = 25

In which layer term “Frames” is used?

Frames are PDU of Data Link Layer

In which layer term “Packets” is used?

Packets are PDU of Network Layer

In which layer term “Segments” is used?

Segments are used at Transport Layer

Give some examples of protocols that work at the Application layer?

Application Layer Protocols are HTTP, HTTPS, Telnet, SSH, DNS, FTP, TFTP, DHCP, RIP

What is CRC? Which layer does CRC work?

Cyclic Redundancy Check is used to detect errors in the network. It works at Data Link Layer (LLC Sub Layer).

What is the purpose of the Data Link?

Data Link Layer is responsible for Framing, Error Detection and Physical Addressing

Which one is reliable – TCP or UDP?

TCP is reliable.

What is the port number of FTP (data) and FTP?

FTP port number 20 (Data); 21 for Control

Which layer provides the logical addressing that routers will use for path determination?

Network Layer

Which layer specifies voltage, wire speed, and pinout cables and moves bits between devices?


Which layer combines bits into bytes and bytes into frames, uses MAC addressing, and provides error detection?

Data Link Layer

Which layer is responsible for keeping the data from different applications separate on the network?

Session layer.

Which layer segments and resembles data into a data stream?

Transport layer.

Which layer provides the physical transmission of the data and handles error notification, network topology, and flow control?

Data Link Layer

Which Layer manages device addressing, tracks the location of devices on the network, and determine the best way to move data?

Network layer.

How Data break down on each layer from top to bottom?

Encapsulation occurs in the following format


MAC address works on which layer? What are the differences between MAC sublayer and LLC sublayer?

MAC works at DATA LINK LAYER. Media Access Control provides physical addressing while Logical Link Control provides error detection, using Ethernet trailer field frame check sequence (FCS). It is 4 bytes field. When a sending device sends data it put the data in a mathematical algorithm and it gets a product, sending device puts the product in FCS. When a receiving device receives data it also put the data in the same mathematical algorithm and gets a product. If both products are the same, Frame is accepted or else discarded.

Which layer is responsible for converting data packets from the Data Link layer into electrical signals?

Physical Layer

At which layer is routing implemented, enabling connections and path selection between two end systems. ?

Network Layer

Which layer defines how data is formatted, presented, encoded, and converted for use on the network?

Presentation Layer

Which layer is responsible for creating, managing, and terminating sessions between applications?

Session Layer

DNS uses which protocol? Why?

DNS uses both TCP and UDP

Which layer is closer to the user?

From the sender’s point of view, the Application Layer is the closest and from the Receiver’s point of view, Physical Layer is the closest.

Differentiate between forward Lookup and reverse lookup in DNS?

Forward Lookup: Name to IP resolution

Reverse Lookup: IP to Name resolution;

What is IPSec?

IPSec provides data security at the IP Packet Level.

What is the way to establish a TCP connection?

TCP Connection is established using a three-way Handshake.

What is the difference between flow control and error control?

Error Controls the process of detecting and correcting both the bit and packet-level error. While flow control is a mechanism to ensure the efficient delivery of Data. Flow control is agreeing on the minimum amount of data that a receiver can handle at a time.


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