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Chapter: Computer Networks : Network Layer

Important Short Questions and Answers: Computer Network Layer

Computer Networks - Network Layer - Important Short Questions and Answers: Computer Networks - Network Layer


1. What are the network support layers and the user support layers? Network support layers:


The network support layers are Physical layer, Data link layer and Network layer. These deals with electrical specifications, physical connection, transport timing and reliability.


User support layers:


The user support layers are: Session layer, Presentation layer, Application layer. These allow interoperability among unrelated software system.


2. With a neat diagram explain the relationship of IEEE Project to the OSI model?

Other layers                                                         Other layers

Network                                                               Network

Logical Link Control                                            Data link

Media Access Control

Physical                                                               Physical


The IEEE has subdivided the data link layer into two sub layers:

·  Logical link control (LLC)

·  Medium access control (MAC)


LLC is non-architecture specific. The MAC sub layer contains a number of distinct modules, each carries proprietary information specific to the LAN product being used.


3. Why IPv6 is preferred than IPv4?

Through IPv6 we can identify more networks or systems than IPv4.


4. Define ICMP.


ICMP uses the source IP address to send the error message to the source of the datagram. ICMP always reports error messages to the original source.


5. Why we migrate from IPv4 to IPv6?


·           Despite all short-term solutions, such as subnetting, classless addressing, and NAT, address depletion is still a long-term problem in the Internet.


·           The Internet must accommodate real-time audio and video transmission. This type of transmission requires minimum delay strategies and reservation of resources not provided in the IPv4 design.


·           The Internet must accommodate encryption and authentication of data for some applications. No encryption or authentication is provided by IPv4.


To overcome these deficiencies, IPv6 (Internetworking Protocol, version 6), also known as IPng (Internetworking Protocol, next generation), was proposed and is now a standard.


6. What is the use of NAT?


NAT (Network Address Translation) is the process where a network device, usually a firewall, assigns a public address to a computer inside a private network.


7. What are the responsibilities of network layer?


The network layer is responsible for the source-to-destination delivery of packet across multiple network links. The specific responsibilities of network layer include the following:

     Logical addressing.



8. What is a virtual circuit?

A logical circuit made between the sending and receiving computers. The connection is made after both computers do handshaking. After the connection, all packets follow the same route and arrive in sequence.


9. What are data grams?


In datagram approach, each packet is treated independently from all others. Even when one packet represents just a place of a multipacket transmission, the network treats it although it existed alone. Packets in this technology are referred to as datagram.


10. What are the two types of implementation formats in virtual circuits?

Virtual circuit transmission is implemented in 2 formats.


·Switched virtual circuit

·Permanent virtual circuit.


11. What is meant by switched virtual circuit?


Switched virtual circuit format is comparable conceptually to dial-up line in circuit switching. In this method, a virtual circuit is created whenever it is needed and exits only for the du ration of specific exchange.


12. What is meant by Permanent virtual circuit?


Permanent virtual circuits are comparable to leased lines in circuit switching. In this method, the same virtual circuit is provided between two uses on a continuous basis. The circuit is dedicated to the specific uses.


13. Define Routers.


Routers relay packets among multiple interconnected networks. They Route packets from one network to any of a number of potential destination networks on internet routers operate in the physical, data link and network layer of OSI model.


14. What is meant by hop count?


The pathway requiring the smallest number of relays, it is called hop-count routing, in which every link is considered to be of equal length and given the value one.


15. How can the routing be classified?

The routing can be classified as,

• Adaptive routing                                     

• Non-adaptive routing.


16. What is time-to-live or packet lifetime?


As the time-to-live field is generated, each packet is marked with a life time, usually the number of h ops that are allowed before a packet is considered lost and accordingly, destroyed. The time-to-live determines the lifetime of a packet.


17. What is meant by brouter?


A brouter is a single protocol or multiprotocol router that sometimes act as a router and sometimes act as a bridge.


18. Write the keys for understanding the distance vector routing.

The three keys for understanding the algorithm are


     Knowledge about the whole networks

     Routing only to neighbors

• Information sharing at regular intervals


19. Write the keys for understanding the link state routing.

The three keys for understanding the algorithm are


• Knowledge about the neighborhood.

• Routing to all neighbors.

• Information sharing when there is a range.


20. How the packet cost referred in distance vector and link state routing?


In distance vector routing, cost refer to hop count while in case o f link state routing, cost is a weighted value based on a variety of factors such as security levels, traffic or the state of the link.


21. How the routers get the information about neighbor?


A router gets its information about its neighbors by periodically sending them a short greeting packet. If the neighborhood responds to the greeting as expected, it is assumed to be alive and functioning. If it does not, a change is assumed to have occurred and the sending router then alerts the rest of the network in its next LSP.


22. What are the four internetworking devices?

The four internetworking devices are,






23. Define IP address.


IP address is the 3-bit number for representing a host or system in the network. One portion of the IP address indicates a networking and the other represents the host in a network.


24. What is Token Bus?


Token Bus is a physical bus that operates as a logical ring using tokens. Here stations are logically organized into a ring. A token is passed among stations. If a station wants to send data, it must wait and capture the token. Like Ethernet, station communicates via a common bus.


25. What is token passing?


Stations may attempt to send data multiple times before a transmission makes it onto a link. This redundancy may create delays of indeterminable length if the traffic is heavy. Token ring resolves this uncertainty by requiring that stations take turns sending data. Each station may transmit only during its turn and may send only one frame during each turn. The mechanism that coordinates this rotation is called token passing.


26. Define Masking?


Masking is the process that extracts the address of the physical network from an IP address.


27. What are the rules of boundary-level masking?

The rules of boundary-level masking


• The bytes in the IP address that corresponds to 255 in the mask will be repeated in the subnetwork address

• The bytes in the IP address that corresponds to 0 in the mask will change to 0 in the subnetwork address


28. What are the rules of nonboundary-level masking?

·  The bytes in the IP address that corresponds to 255 in the mask will be repeated in the subnetwork address


·The bytes in the IP address that corresponds to 0 in the mask will change to 0 in the subnetwork address


·           For other bytes, use the bit-wise AND operator


29. Define Gateway.


A device used to connect two separate networks that we different communication protocols.


30. What is LSP?


In link state routing, a small packet containing routing information sent by a router to all other router by a packet called link state packet




Address Resolution Protocol (ARP): In TCP/IP, a protocol for obtaining the physical addressof a node when the Internet address is known.


Anycast Address: An address that defines a group of computers with addresses that have thesame beginning.


Base Header: In IPv6, the main header of the datagram.


Broadcast Address: An address that allows transmission of a message to all nodes of a network.


Broadcasting: Transmission of a message to all nodes in a network.


Checksum:A value used for error detection. It is formed by adding data units using one'scomplement arithmetic and then complementing the result.


Classful Addressing: An IPv4 addressing mechanism in which the IP address space is dividedinto 5 classes: A, B, C, D, and E. Each class occupies some part of the whole address space.


Classless Addressing: An addressing mechanism in which the IP address space is not dividedinto classes.


Client Process: A running application program on a local site that requests service from arunning application program on a remote site.


Compatible Address: An IPv6 address consisting of 96 bits of zero followed by 32 bits of IPv4.


Core-Based Tree (CBT): In multicasting, a group-shared protocol that uses a center router asthe root of the tree.


Datagram Network:A packet-switched network in which packets are independent from eachother.


Differentiated Services (DS or Diffserv): A class-based QoS model designed for IF.


Dijkstra's algorithm: In link state routing, an algorithm that finds the shortest path to otherrouters.


Direct Delivery: A delivery in which the final destination of the packet is a host connected tothe same physical network as the sender.


Distance Vector Multicast Routing Protocol (DVMRP): A protocol based on distance vectorrouting that handles multicast routing in conjunction with IGMP.


Distance Vector Routing: A routing method in which each router sends its neighbors a list ofnetworks it can reach and the distance to that network.


Dynamic Host Configuration Protocol (DHCP): An extension to BOOTP that dynamicallyassigns configuration information.


Firewall: A device (usually a router) installed between the internal network of an organizationand the rest of the Internet to provide security.


Forwarding: Placing the packet in its route to its destination.


Fragmentation: The division of a packet into smaller units to accommodate a protocol's MTU.


Frame: A group of bits representing a block of data.


Integrated Services (IntServ): A flow-based QoS model designed for IP.


Internet address: A 32-bit or l28-bit network-layer address used to uniquely define a host on aninternet using the TCP/IP protocol.


Internet Control Message Protocol (ICMP): A protocol in the TCP/IP protocol suite thathandles error and control messages.


Internet Control Message Protocol, version 6 (ICMPv6): A protocol in IPv6 that handleserror and control messages.


Internet Group Management Protocol (IGMP): A protocol in the TCP/IP protocol suite thathandles multicasting.


Internet Protocol (lP): The network-layer protocol in the TCP/IP protocol suite governingconnectionless transmission across packet switching networks.


Internet Protocol version 4 (IPv4): The current version of Internet Protocol.


Internet Protocol, version 6 (IPv6): The sixth version of the Internet Protocol.


Link State Routing: A routing method in which each router shares its knowledge of changes inits neighborhood with all other routers.


Mapped Address: An IPv6 address used when a computer that has migrated to IPv6 wants tosend a packet to a computer still using IPv4.


Multicast Routing: Moving a multicast packet to its destinations.


Multiple Unicasting:Sending multiple copies of a message, each with a different unicastaddress.


Network Address Translation (NAT): A technology that allows a private network to use a setof private addresses for internal communication and a set of global Internet addresses for external communication.


Open Shortest Path First (OSPF): An interior routing protocol based on link state routing.


Path Vector Routing: A routing method on which BGP is based; in this method, the ASSthrough which a packet must pass is explicitly listed.


Protocol Independent Multicast (PIM): A multicasting protocol family with two members,PIM-DM and PIM-SM; both protocols are unicast-protocol dependent.


Protocol Independent Multicast, Dense Mode (PIM-DM): A source-based routing protocolthat uses RPF and pruning/grafting strategies to handle multicasting.


Protocol Independent Multicast, Sparse Mode (PIM-SM): A group-shared routing protocolthat is similar to CBT and uses a rendezvous point as the source of the tree.


Reverse Address Resolution Protocol (RARP): A TCPIIP protocol that allows a host to find itsInternet address given its physical address.


Unicast Address: An address belonging to one destination.


Unicast Routing: The sending of a packet to just one destination.


Unicasting: The sending of a packet to just one destination.


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