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OSI Model Interview Questions and Answers [CCNA]

List the Layers of OSI Model?
Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer, Data Link Layer, Physical Layer.

What are the Functions of Transport, Network and Data Link Layer?
Transport layer
1. It segments and reassemble data from upper-layer applications and combines it into the same data stream.
2. It provides end-to-end data transport services.
3. Establishes the logical connection between the sending host and destination host in an internetwork.
4. It ensures Data integrity at the Transport layer by maintaining flow control.

Network layer
1. The Network layer (layer 3) manages device addressing.
2. It tracks the location of devices on the network.
3. It determines the best way to move data between devices that are not locally attached.
4. Routers Functions at the Network layer to provide the routing services within an internetwork.

Data link layer
1. The Data Link layer is responsible for the physical transmission of the data.
2. It handles error notification, flow control.
3. Data Link layer ensures that messages are delivered to the proper device on a LAN using mac addresses.
4. It translates messages from the Network layer into bits for the Physical layer to transmit.
5. The Data Link layer formats the message into the data frame and adds a customized header containing the hardware destination and source address.

Which Layer is responsible for Reliable connection?
Transport Layer

What are the different protocols works at each of the layers in OSI Model?
Physical Layer
ISDN (Integrated Services Digital Network), ADSL (Asymmetric Digital Subscriber Line), Universal Serial Bus, Bluetooth, Controller Area Network, Ethernet.

Data Link layer
Spanning Tree Protocol, VLan Trunking Protocol, Dynamic Trunking Protocol, HDLC, PPP, Frame Relay, Token Ring.

Network Layer

Transport Layer

Session Layer
NFS (Network File System).

Presentation Layer
Data encryption/decryption, Data compression, Data Conversion Protocols

Application Layer

What is a port number and give some examples?
TCP & UDP must use port numbers to communicate with upper layers because these are what keep track of different conversations crossing the network simultaneously.

                        PROTOCOLS                           PORT NUMBERS
                        FTP                       20, 21
                        TELNET                       23
                        SMTP                       25
                        DNS                       53
                       DHCP                       67 (DHCP Server)
                       68 (DHCP Client)
                       TFTP                       69
                       HTTP                       80
                       POP3                      110
                       NTP                      123
                       IMAP4                      143
                       SNMP                      161
                       BGP                      179
                       HTTPS                      443
                       RIP                      520

What is the Range Of Port Numbers ?
Well Known Ports  -  0 to 1023
Registered Ports     -  1024 to 49151
Open Ports              -  49152 to 65535

What is a Protocol Number and give some examples?
In IPV4 There is a Field called Protocol to identify the Next Level Protocol. In IPV6 this Field is called "Next Header" Field.                      

                        PROTOCOL                         PROTOCOL NUMBER
                        ICMP                                        1
                        IGMP                                        2
                        IPV4                                        4
                        TCP                                        6
                        EGP                                        8
                        IGP                                        9
                        UDP                                       17
                        IPV6                                       41
                        GRE                                       47
                        EIGRP                                       88
                        OSPF                                       89
                        VRRP                                      112

Define Unicast, Multicast and Broadcast?
Broadcast is the term used to describe communication where a piece of information is sent to all nodes on the network.
Multicast is the term used to describe communication where a piece of information is sent from a single source and transmitted to many devices but not all devices.
Unicast is the term used to describe communication where a piece of information is sent to a single destination host.

What is the difference between Half-duplex and Full-duplex?
Half Duplex - Data can Flow in both Direction but not simultaneously. At a time Data can flow only in one direction. example - HUB.
Full Duplex - Data can Flow in both Direction Simultaneously. example - Switch.

What is the MAC format?
It is a 12 Digits 48 Bit (6 Byte) Hardware address written in Hexadecimal Format.
It consists of two parts -
The First 24 Bits OUI (Organizationally Unique Identifier) is assigned by IEEE.
The Last 24 Bits is Manufacturer-assigned Code.

What is a Frame?
The Data Link layer formats the message into pieces, each called a data frame, and adds a customized header containing the hardware source and destination address.

What is the TCP/IP Model?
TCP/IP is four layer standard model.
The four layers of TCP/IP model are Application layer, Transport layer, Internet layer, Network access layer.

What are the protocols that are included by each layer of the TCP/IP model?                                                              

Layers of the TCP/IP model Protocols
Application Layer  DNS, DHCP, FTP, TFTP, SMTP, HTTP, Telnet, SSH
Transport LayerTCP, UDP
Internet layer Layer   IP, ICMP, IGMP
Network access layerEthernet, Token Ring, FDDI, X.25, Frame Relay, ARP, RARP


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