Master of Computer Application (MCA) – Semester 3MC0075 – Computer Networks

Assignment Set – 1

Que 1. Describe the theory of network software in computer networks.

Ans:-

What is a Network:-

The term “Computer Network” is used to mean an interconnected collection of autonomous computers. Two computers are said to be interconnected, if they are able to exchange information. The connection can be wired or wireless. A large computer with remote printers and terminals is not a network. Nor a system with one control and many slaves is a network. With a network the user must explicitly log into a machine, explicitly submit jobs remotely, explicitly move files around and generally handle all network management personally.

Distribution or exchange of information is very important for any group activity to work or function efficiently and effectively. This is best achieved by networking. Computer network have become an important tool of communication, with an added advantage of being able to store large amount of data and also being able to deliver it as and when demanded. A computer network finds application in almost every field of life.

A computer network can do many things, which are listed below, that a stand-alone machine cannot do:

1. Share a single copy of a file among all the users, even simultaneously.

2. Transfer files from computer to computer or from one computer to the central hard disk on the server.

3. Share applications over the entire network, thus reducing the cost of purchasing the multiple copies. A multi-user license costs much less than buying a stand alone license.

4. Share resources like printers, hard disks, CD-ROM drives, modems, fax machines etc.

5. Use e-mail for internal communications, inert office notes etc., creating an almost paper less office.

Components of a computer network

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A network is a combination of different components. The most essential components of a simple network are listed below:

· Servers: Servers are faster computers that run various software’s, store and process information and also provide a human interface for the users to be able to use the networked computers.

· Nodes: Nodes are the computers on the network, which are provided to the users to carry out their tasks using the network.

· Workstation: A node, which is more powerful, and can handle local information processing or graphics processing is called a workstation. The workstation works only for the person sitting in front of it, where as a server serves all the people on the network to share its resources. A workstation usually has an inexpensive, small hard disk to carry out local tasks. Some workstations, called diskless workstations, have no disk drive of their own. Such workstations also called dumb terminals and they rely completely on the LAN for their access. The network operating system lets the nodes work as if all the resources at the server belong to the node itself.

· Network Operating System (NOS): The network requires some software to control all the information transfer activity on the network, like the traffic police to control the traffic. The software called NOS handles these tasks. Networks, which are more complex, require network devices like hubs, switches & routers to carry out different network functions.

· LAN Software: On the network, each computer is called a node or a workstation unless there are certain computers designed as servers. LAN cables connect all the nodes and servers together to form the network. In addition to its local disk operating system, each node requires networking software that enable the nodes to communicate with the servers. In return, the file servers run network software that communicates with the nodes.

· LAN Cable: This is the medium or channel over which the information travels from computer to computer. The information travels from one computer onto the medium and then from the medium to another computer in the form that it can be read.

· Network Interface Card: Each computer contains a network interface card. This card is used to connect the cables to the computers. These cards help the computer to transfer the data at a faster rate and in the form of packets. These cards are plugged into the computer motherboard. These cards are generally called as Ethernet cards.

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Que 2. Describe the OSI reference model and compare it with TCP / IP model.

Ans:-

There are two important network architectures. They are ISO-OSI reference model and TCP/IP reference model.

In 1977, the International Organization for Standardization (ISO) began to develop its OSI networking suite. OSI has two major components: an abstract model of networking (the Basic Reference Model, or seven-layer model), and a set of concrete protocols. The standard documents that describe OSI are for sale and not currently available online.

Parts of OSI have influenced Internet protocol development, but none more than the abstract model itself, documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.

In particular, Internet protocols are deliberately not as rigorously architected as the OSI model, but a common version of the TCP/IP model splits it into four layers. The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its End-to-End Layer includes the graceful close function of the OSI Session Layer as well as the Transport Layer. Its Internet work Layer is equivalent to the OSI Network Layer, while its Interface layer includes the OSI Data Link and Physical Layers. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the Internal Organization of the Network Layer document.

Protocols enable an entity in one host to interact with a corresponding entity at the same layer in a remote host. Service definitions abstractly describe the functionality provided to a (N)-layer by an (N-1) layer, where N is one of the seven layers inside the local host

The OSI Reference Model

This reference model is proposed by International standard organization (ISO) as a a first step towards standardization of the protocols used in various layers in 1983 by Day and Zimmermann. This model is called Open system Interconnection (OSI) reference model. It is referred OSI as it deals with connection open systems. That is the systems are open for communication with other systems. It consists of seven layers.

Layers of OSI Model

The principles that were applied to arrive at 7 layers:

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1. A layer should be created where a different level of abstraction is needed.

2. Each layer should perform a well defined task.

3. The function of each layer should define internationally standardized protocols

4. Layer boundaries should be chosen to minimize the information flow across the interface.

5. The number of layers should not be high or too small.

Figure 2.5: ISO – OSI Reference Model

The ISO-OSI reference model is as shown in figure 2.5. As such this model is not a network architecture as it does not specify exact services and protocols. It just tells what each layer should do and where it lies. The bottom most layer is referred as physical layer. ISO has produced standards for each layers and are published separately.

A Comparison of OSI and TCP/IP Reference Models

Concepts central to the OSI model are:

· Services: It tells what the layer does.

· Interfaces: It tells the processes above it how to access it. It specifies what parameters are and what result to expect.

· Protocols: It provides the offered service. It is used in a layer and are layers own business.

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The TCP/IP did not originally distinguish between the service, interface & protocols. The only real services offered by the internet layer are SEND IP packets and RECEIVE IP packets.

The OSI model was devised before the protocols were invented. Data link layer originally dealt only with point-to-point networks. When broadcast networks came around, a new sub-layer had to be hacked into the model. With TCP/IP the reverse was true, the protocols came first and the model was really just a description of the existing protocols. This TCP/IP model did fit any other protocol stack.

Then OSI model has seven layers and TCP/IP has four layers as shown in figure 2.11

Figure 2.11: Comparisons of the two reference models

Another difference is in the area of connectionless and connection oriented services. The OSI model supports both these services in the network layer but supports only connection oriented communication in the transport layer. Where as the TCP/IP has supports only connection less communication in the network layer, and supports both these services in the transport layer.

A Critique of the OSI Model and Protocols

Why OSI did not take over the world

· Bad timing

· Bad technology

· Bad implementations

· Bad politics

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Que.3. Explain Circuit, Message and Packet switching techniques.

Ans:- Switching

For transmission of data beyond local area, communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes. The switching nodes are not concerned with the content of data. Rather their purpose is to provide a switching facility that will move the data from node to node until it reaches the destination. Circuit switching and packet switching techniques are more commonly used and are as shown in Figure 4.5 (a) and (b) respectively.

Circuit switching

A circuit switching network is one that establishes a dedicated circuit (or channel) between nodes and terminals before the users may communicate. Each circuit that is dedicated cannot be used by other callers until the circuit is released and a new connection is set up. Even if no actual communication is taking place in a dedicated circuit then, that channel still remains unavailable to other users. Channels that are available for new calls to be set up are said to be idle. Circuit switching is used for ordinary telephone calls. It allows communications equipment and circuits, to be shared among users. Each user has sole access to a circuit (functionally equivalent to a pair of copper wires) during network use.

Figure 4.5: (a) circuit switching (b) packet switching

For call setup and control (and other administrative purposes), it is possible to use a separate dedicated signalling channel from the end node to the network. ISDN is one such service that uses a separate signalling channel. The method of establishing the connection and monitoring its progress and termination through the network may also utilize a separate control channel.

Circuit switching can be relatively inefficient because capacity is wasted on connections which are set up but are not in continuous use (however momentarily). On the other hand, the connection is immediately available and capacity is guaranteed until the call is disconnected

Communication using circuit switching involves three phases discussed below:

1. Connection establishment: Before any signal can be transmitted, an end to end circuit must be established.

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2. Data transfer: Information can now be transmitted from source through the network to the destination using the dedicated path established.

3. Termination: After some period of data transfer, the connection is terminated

Consider communication between two points A and D in a network as shown in fig. 4.6. The connection between A and D is provided using (shared) links between two other pieces of equipment, B and C.

Figure: A four node and 3 link network

Network use is initiated by a connection phase, during which a circuit is set up between source and destination, and terminated by a disconnect phase as listed above. These phases, with associated timings, are illustrated in the figure 4.7.

Figure: A circuit switched connection between A and D

(Information flows in two directions. Information sent from the calling end is shown in grey and information returned from the remote end is shown in black)

After a user requests a circuit, the desired destination address must be communicated to the local switching node (B). In a telephony network, this is achieved by dialing the number. Node B receives the connection request and identifies a path to the destination (D) via an intermediate node (C). This is followed by a circuit connection phase handled by the switching nodes and initiated by allocating a free circuit to C (link BC), followed by transmission of a call request signal from node B to node C. In turn, node C allocates a link (CD) and the request is then passed to node D after a similar delay.

The circuit is then established and may be used. While it is available for use, resources (i.e. in the intermediate equipment at B and C) and capacity on the links between the equipment are dedicated to the use of the circuit.

After completion of the connection, a signal confirming circuit establishment (a connect signal in the diagram) is returned; this flows directly back to node A with no search delays

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since the circuit has been established. Transfer of the data in the message then begins. After data transfer, the circuit is disconnected; a simple disconnect phase is included after the end of the data transmission.

Delays for setting up a circuit connection can be high, especially if ordinary telephone equipment is used. Call setup time with conventional equipment is typically on the order of 5 to 25 seconds after completion of dialing. New fast circuit switching techniques can reduce delays. Trade-offs between circuit switching and other types of switching depend strongly on switching times.

Message switching

Message switching was the precursor of packet switching, where messages were routed in their entirety and one hop at a time. It was first introduced by Leonard Kleinrock in 1961. Message switching systems are nowadays mostly implemented over packet-switched or circuit-switched data networks

Hop-by-hop Telex forwarding are examples of message switching systems. E-mail is another example of a message switching system. When this form of switching is used, no physical path is established in advance in between sender and receiver. Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e. router) then forwarded later at one hop at a time.

Each block is received in its entity form, inspected for errors and then forwarded or re-transmitted. It is a form of store-and-forward network. Data is transmitted into the network and stored in a switch. The network transfers the data from switch to switch when it is convenient to do so, as such the data is not transferred in real-time. Blocking can not occur, however, long delays can happen. The source and destination terminal need not be compatible, since conversions are done by the message switching networks.

Again consider a connection of a network shown in figure 4.6. For instance, when a telex (or email) message is sent from A to D, it first passes over a local connection (AB). It is then passed at some later time to C (via link BC), and from there to the destination (via link CD). At each message switch, the received message is stored, and a connection is subsequently made to deliver the message to the neighboring message switch. Message switching is also known as store-and-forward switching since the messages are stored at intermediate nodes en route to their destinations.

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Figure: Message switching to communicate between A and D

The figure 4.8 illustrates message switching; transmission of only one message is illustrated for simplicity. As the figure indicates, a complete message is sent from node A to node B when the link interconnecting them becomes available. Since the message may be competing with other messages for access to facilities, a queuing delay may be incurred while waiting for the link to become available. The message is stored at B until the next link becomes available, with another queuing delay before it can be forwarded. It repeats this process until it reaches its destination.

Circuit setup delays are replaced by queuing delays. Considerable extra delay may result from storage at individual nodes. A delay for putting the message on the communications link (message length in bits divided by link speed in bps) is also incurred at each node enroute. Message lengths are slightly longer than they are in circuit switching, after establishment of the circuit, since header information must be included with each message; the header includes information identifying the destination as well as other types of information. Most message switched networks do not use dedicated point-to-point links.

Packet switching

Packet switching splits traffic data (for instance, digital representation of sound, or computer data) into chunks, called packets. Packet switching is similar to message switching. Any message exceeding a network-defined maximum length is broken up into shorter units, known as packets, for transmission. The packets, each with an associated header, are then transmitted individually through the network. These packets are routed over a shared network. Packet switching networks do not require a circuit to be established and allow many pairs of nodes to communicate almost simultaneously over the same channel. Each packet is individually addressed precluding the need for a dedicated path to help the packet find its way to its destination.

Packet switching is used to optimize the use of the channel capacity available in a network, to minimize the transmission latency (i.e. the time it takes for data to pass across the network), and to increase robustness of communication.

Again consider the same network as shown in figure 4.6. Now the message of figure 4.8 is broken into three small units called packets and labeled 1-3 as illustrated in figure 4.9.

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Figure :Packet-switched communication between A and D

The most well-known use of packet switching is the Internet. The Internet uses the Internet protocol suite over a variety of data link layer protocols. For example, Ethernet and Frame relay are very common. Newer mobile phone technologies (e.g., GPRS, I-mode) also use packet switching. Packet switching is also called connectionless networking because no connections are established

There are two important benefits from packet switching.

1. The first and most important benefit is that since packets are short, the communication links between the nodes are only allocated to transferring a single message for a short period of time while transmitting each packet. Longer messages require a series of packets to be sent, but do not require the link to be dedicated between the transmission of each packet. The implication is that packets belonging to other messages may be sent between the packets of the message being sent from A to D. This provides a much fairer sharing of the resources of each of the links.

2. Another benefit of packet switching is known as "pipelining". Pipelining is visible in the figure above. At the time packet 1 is sent from B to C, packet 2 is sent from A to B; packet 1 is sent from C to D while packet 2 is sent from B to C, and packet 3 is sent from A to B, and so forth. This simultaneous use of communications links represents a gain in efficiency; the total delay for transmission across a packet network may be considerably less than for message switching, despite the inclusion of a header in each packet rather than in each message.

The long-haul circuit-switching telecommunications network was originally designed to handle voice traffic, and the majority of traffic on these networks continues to be voice. A key characteristic of circuit-switching networks is that resources within the network are dedicated to a particular call. For voice connections, the resulting circuit will enjoy a high percentage of utilization because; most of the time, one party or the other is talking.

However, as the circuit-switching network began to be used increasingly for data connections, two shortcomings became apparent:

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· In a typical user/host data connection (e.g., personal computer user logged on to a database server), much of the time the line is idle. Thus, with data connections, a circuit-switching approach is inefficient.

· In a circuit-switching network, the connection provides for transmission at a constant data rate. Thus, each of the two devices that are connected must transmit and receive at the same data rate as the other. This limits the utility of the network in interconnecting a variety of host computers and workstations.

To understand how packet switching addresses these problems, let us briefly summarize packet-switching operation. Data are transmitted in short packets. A typical upper bound on packet length is 1000 octets (bytes).If a source has a longer message to send, the message is broken up into a series of packets. Each packet contains a portion (or all for a short message) of the user’s data plus some control information. The control information at a minimum includes the information that the network requires to be able to route the packet through the net- work and deliver it to the intended destination. At each node en route, the packet is received stored briefly, and passed on to the next node.

Now, let’s consider figure 4.10, assuming that it depicts a simple packet switching network. Consider a packet to be sent from station A to station E. The packet includes control information that indicates that the intended destination is E. The packet is sent from A to node 4. Node 4 stores the packet, determines the next leg of the route (say 5), and queues the packet to go out on that link (the 4-5 link). When the link is available, the packet is transmitted to node 5, which forwards the packet to node 6, and finally to E. This approach has a number of advantages over circuit switching:

Figure :The Use of Packets

· Line efficiency is greater, as a single node-to-node link can be dynamically shared by many packets over time. The packets are queued up and transmitted as rapidly as possible over the link. By contrast, with circuit switching, time on a node-to-node link is pre-allocated using synchronous time division multiplexing. Much of the time, such a link may be idle because a portion of its time is dedicated to a connection that is idle.

· A packet-switching network can perform data-rate conversion. Two stations of different data rates can exchange packets because each connects to its node at its proper data rate.

· When traffic becomes heavy on a circuit-switching network, some calls are blocked; that is, the network refuses to accept additional connection requests until the load on the network decreases. On a packet-switching network, packets are still accepted, but delivery delay increases.

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· Priorities can be used. Thus, if a node has a number of packets queued for transmission, it can transmit the higher-priority packets first. These rackets will therefore experience less delay than lower-priority packets.

Que 4.Explain the following concepts of Internetworking: A) Internet architecture B) Protocols and Significance for internetworking C) Internet layering model

Ans:-

Internet architecture:- The Internet is a worldwide, publicly accessible network of interconnected computer networks that transmit data by packet switching using the standard Internet Protocol (IP). It is a "network of networks" that consists of millions of smaller domestic, academic, business, and government networks, which together carry various information and services, such as electronic mail, online chat, file transfer, and the interlinked web pages and other documents of the World Wide Web.

How are networks interconnected to form an internetwork? The answer has two parts. Physically, two networks can only be connected by a computer that attaches both of them. But just a physical connection cannot provide interconnection where information can be exchanged as there is no guarantee that the computer will cooperate with other machines that wish to communicate.

Internet is not restricted in size. Internets exist that contain a few networks and internets also exist that contain thousands of networks. Similarly the number of computers attached to each network in an internet can vary. Some networks have no computers attached, while others have hundreds.

To have a viable internet, we need a special computer that is willing to transfer packets from one network to another. Computers that interconnect two networks and pass packets from one to the other are called internet gateways or internet routers.

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Fig.: Overview of Internet

Figure 1.1 illustrates the overview of internet architecture. Let us assume a client calls his/her Internet Service provider (ISP) over a dial up telephone line. The modem is a card within a PC that converts the digital signals the computer produces to analog signals that can pass over telephone system. These signals are transferred to the ISP’s point of presence (POP), where they are removed from the telephone system and injected into the ISP’s regional network. From this point onwards, the system is fully digital and packet switched.

The ISP’s regional network consists of interconnected routers in the various cities the ISP serves. If the packet is destined for a host served directly by the ISP, then the packet is delivered to the host. Otherwise, it is handed over to the ISP’s backbone operator.

At the top we have companies like AT&T, Sprint, that are major backbone operators. They operate large international backbone networks with thousands of routers connected by high bandwidth optical fibers. Large companies run server farms often connect directly to the backbone. Backbone operators encourage this direct connection by renting space called Carrier hotels.

If a packet given to the backbone is destined for an ISP or company served by the backbone, it is sent to the closet router and handed off there. Many backbones of varying sizes exist in the world. In such cases the packet may have to be sent to a competing backbone. To allow packets to hop between backbones, all major backbones connect at the network access point (NAP). Basically a NAP is a room full of routers, at least one per backbone. A LAN in a room connects all these routers, so packets can be forwarded from any backbone to any other backbone. In addition to being interconnected at NAP’s, the larger backbones have numerous direct connections between their routers, a technique known as private peering.

An organization uses single router to connect its entire network. There are two reasons

· Because the CPU and memory in a router are used to process each packet, the processor in one router is insufficient.

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· Redundancy improves internet reliability. Protocol software continuously monitors internet connections and instructs the routers to send traffic along alternative paths when a network or router fails.

Hence when planning an internet, an organization must choose a design that meets its need for reliability, capacity, and cost. In particular, the exact details of the expected traffic, the organization’s reliability requirements, internet topology, that often depends on the bandwidth of the physical networks and finally the cost of available router hardware.

Protocols for internetworking :- Many protocols have been used for use in an internet. One suite known as The TCP/IP internet protocol stands out most widely used for internets. Most networking professional simply refer this protocol as TCP/IP. Work on the transmission control protocol (TCP) began in the 1970’s. The U.S military funded the research in TCP/IP and internetworking through the Advanced Research Projects Agency in short known as ARPA.

1. Significance of internetworking and TCP/IP

Internetworking has become one of the important technique in the modern networking. Internet technology has revolutionized the computer communication. The TCP/IP technology has made possible a global Internet, which reaches millions of schools, commercial organizations, government and military etc around the world.

The worldwide demand for internetworking products has affected most companies sell networking technologies. Competition has increased among the companies that sell the hardware and software needed for internetworking. Companies have extended the designs in two ways

· The protocols have adapted to work with many network technologies

· And new features have been adapted that allow the protocols to transfer data across the internets

Internet Layering Model :-

Internet uses the TCP/IP reference model. This model is also called as Internet layering model or internet reference model. This model consists of 5 layers as illustrated in figure 1.3.

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Fig. 1.3: The five layers of TCP/IP reference model

A goal was of continuing the conversation between source and destination even if transmission went out of operation. The reference model was named after two of its main protocols, TCP (Transmission Control Protocol) and IP (Internet Protocol). The purpose of each layer of TCP/IP is given below:

Layer 1: Physical layer

This layer corresponds to basic network hardware

Layer 2: Network interface

This layer specifies how to organize data into frames and how a computer transfers frames over a network. It interfaces the TCP/IP protocol stack to the physical network.

Layer 3: Internet

This layer specifies the format of packets sent across an internet. It also specifies the mechanism used to forward packets from a computer through one or more routers to the final destination.

Layer 4: Transport

This layer deals with opening and maintaining connections, ensuring that packets are in fact received. The transport layer is the interface between the application layer and the complex hardware of the network. It is designed to allow peer entities on the source and destination hosts to carry on conversations.

Layer 5: Network interface

Each protocol of this layer specifies how one application uses an internet.

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Que5. Explain the following different classes of IP addresses:

A) Primary classful addresses B) Class A C) Class B D) Class C

Ans :- Primary addresses :-

Out of five the three classes are called Class A, Class B, and Class C. These three classes together are often referred to as "classful" addressing or primary address class.

Fig. 2.1(a): Principle Classful IP Address Formats

Each class fixes the boundary between the network-prefix and the host-number at a different point within the 32-bit address. The formats of the fundamental address classes are illustrated in Figure 2.1(a). One of the fundamental features of classful IP addressing is that each address contains a self-encoding key that identifies the dividing point between the network-prefix and the host-number.

2.4.2 Class A Networks (/8 Prefixes)

Each Class A network address has an 8-bit network-prefix with the highest order bit set to 0 and a seven-bit network number, followed by a 24-bit host-number. Today, it is no longer considered ‘modern’ to refer to a Class A network. Class A networks are now referred to as "/8s" (pronounced "slash eight" or just "eights") since they have an 8-bit network-prefix.

A maximum of 126 (2 7 -2) /8 networks can be defined as shown in figure 2.1(b). The calculation requires that the 2 is subtracted because the /8 network 0.0.0.0 is reserved for use as the default route and the /8 network 127.0.0.0 (also written 127/8 or 127.0.0.0/8) has been reserved for the "loop back" function. Each /8 supports a maximum of 16,777,214 (2 24 -2) hosts per network. The host calculation requires that 2 is subtracted because the all-0s ("this network") and all-1s ("broadcast") host-numbers may not be assigned to individual hosts.

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Since the/8 address block contains 231 (2,147,483,648) individual addresses and the IPv4 address space contains a maximum of 2 32 (4,294,967,296) addresses, the /8 address space is 50% of the total IPv4 unicast address space.

2.4.3 Class B Networks (/16 Prefixes)

Each Class B network address has a 16-bit network-prefix with the two highest order bits set to 1-0 and a 14-bit network number, followed by a 16-bit host-number as illustrated in figure 2.1(b). Class B networks are now referred to as"/16s" since they have a 16-bit network-prefix.

A maximum of 16,384 (2 14) /16 networks can be defined with up to 65,534 (2 16 -2) hosts per network. Since the entire /16 address block contains 2 30 (1,073,741,824) addresses, it represents 25% of the total IPv4 unicast address space.

2.4.4 Class C Networks (/24 Prefixes)

Each Class C network address has a 24-bit network-prefix with the three highest order bits set to 1-1-0 and a 21-bit network number, followed by an 8-bit host-number as shown in figure 2.1(b). Class C networks are now referred to as "/24s" since they have a 24-bit network-prefix. A maximum of 2,097,152 (2 21) /24 networks can be defined with up to 254 (2 8 -2) hosts per network. Since the entire /24 address block contains 2 29 (536,870,912) addresses, it represents 12.5% (or 1/8th) of the total IPv4 unicast address space.

Que:- 6. Discuss the theory and practical applications of Supernetting.

Ans: - Supernetting

Subnetting techniques could not prevent internet growth from eventually exhausting address space. It replaced the previous generation of IP address syntax, classful networks. It allowed increased flexibility when dividing ranges of IP addresses into separate networks. Instead of using a single IP network prefix for multiple physical networks at a given organization, supernetting allows the address assigned to a single organization to span multiple classed prefixes. Thus the network/host division can occur at any bit boundary in the address. The process can be recursive, with a portion of the address space being further divided into even smaller portions, through the use of masks which cover more bits.

The facts why classless addressing was adopted:

1. The classful scheme did not divide network addresses into classes equally. Although less than 17,000 class B numbers can be assigned, more than 2 million class C network numbers do exist.

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2.

3. Class C numbers being requested slowly only a small percent of them had been assigned.

4. Class B prefixes would be exhausted quickly. This situation was known as Running Studies showed that at the rate class B networks were being assigned, Out of Address Space (ROADS) problem.

Classless addressing is also called supernet addressing or supernetting. It provides:

· More efficient use of increasingly scarce IPv4 addresses.

· Greater use of hierarchy in address assignments (prefix aggregation), lowering the overhead of the Internet-wide inter-domain routing.

Consider a medium-sized organization that joins the Internet. Such an organization would prefer to use a single class B address for two reasons:

· A class C address cannot accommodate more than 254 hosts and

· A class B address has sufficient bits to make subnetting convenient.

Effect of supernetting on routing

Allocating many class C addresses solves the immediate problem of address space exhaustion. However it suffers from the drawback that the information that the routers store and exchange increases dramatically. Hence for routing a technique known as Classless Inter-Domain Routing (CIDR).

Classless Inter-Domain Routing (CIDR) was introduced in 1993 and is the latest refinement to the way IP addresses are interpreted. CIDR scheme takes an approach that is complement of subnet addressing. CIDR is principally a bitwise, prefix-based standard for the interpretation of IP addresses. It facilitates routing by allowing blocks of addresses to be grouped together into single routing table entries. These groups, commonly called CIDR blocks, share an initial sequence of bits in the binary representation of their IP addresses.

That is CIDR collapses a block of contiguous class C addresses into a single entry represented by a pair:

(Network address, count)

Where, network address is the smallest address in the block. For example the pair:

(192.5.48.0, 3)

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This example indicates that the there are 3 network addresses and they are 192.5.48.0, 192.5.49.0, 192.5.50.0

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