February 2010

Master of Computer Application (MCA) – Semester 3

MC0075 – Computer Networks

Assignment Set - 1

1. Describe the classification of Computer Networks giving suitable real time

examples for each class.Ans –

Classification of Computer Networks

There is no generally taxonomy accepted into which all computer networks fit. The computer networks are classified depending on transmission technology and scale.

Computer networks may be classified according to the network layer at which they operate according to some basic reference models that are considered to be standards in the industry such as the seven layer OSI reference model and the four layers TCP/IP model. Few ways of classification are listed below.

· By transmission techniques: Computer networks may be classified as broadcast or point to point networks.

· By scale: to the scale or extent of reach of the network, for example as a Personal area network (PAN), Local area network (LAN), Campus area network (CAN), Metropolitan area network (MAN), or Wide area network (WAN).

· By connection method: Computer networks may be classified according to the technology that is used to connect the individual devices in the network such as HomePNA, Power line communication, Ethernet, or Wireless LAN.

· By functional relationship: Computer networks may be classified according to the functional relationships which exist between the elements of the network, for example Active Networking, Client-server and Peer-to-peer (workgroup) architectures.

· By network topology: Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc. This topic is covered in detail in unit 4.

· By services provided: Computer networks may be classified according to the services which they provide, such as Storage area networks, Server farms, Process control networks, Value-added network, Wireless community network, etc.

· By Protocol: Computer networks may be classified according to the communications protocol that is being used on the network. Read the articles on List of network protocols and protocol stacks for more information.

The most common way of classifications is by transmission techniques, by scale and by the way the computers are connected. The first two ways are discussed in the following section where as the classification by topology is discussed in unit 4.

Based on transmission technology

· Broadcast links

· Point-to-point links

Broadcast networks have a single communication channel that is shared by all the users on the network. Short messages are commonly called as packets or frames (in certain context). The user on the network sends packets. All other machines receive these packets. An address field within the packet or frame specifies the address of the destination machine. So upon receiving the packet, all machines check the address field. Only intended user uses or processes the packet or frame and others neglect and discard it. As an example in a class of 50 students, the teacher puts question to say ‘X’ student (where X is the name of the student). All the students hear to the question but will not answer as the question is intended to X only. Hence only X will analyze the question and others will not respond.

Broadcast system generally allows the possibility of addressing a packet to all the destinations by using a special code in the address field. When this code is transmitted, it is received and processed by every machine on the network. Again considering the above example: A teacher put forth the question in a class to all students. That is the teacher does not ask to a specific student by any unique name. Then, all are supposed to analyze the question and answer. This mode of operation is referred to broadcasting. Some broadcasting systems also support transmission to a subset of the users, which is a group of users. This mode is called as multicasting.

In contrast the point-to-point network consists of many connections between individual pairs of machines. A packet to be sent from source to destination may have to first visit one or more intermediate machines. Usually different routes of different length are possible. So finding the best path or route is important in point-to-point networks. This type of transmission with one sender and one receiver is also referred to as unicasting.

Geographically localized networks or smaller networks tend to use broadcasting where as larger networks usually are point-to-point networks.

Based on the their scale

At the top we have personal area networks (PAN), networks meant for a single person. For example a wireless network connecting a computer with its mouse, keyboard and a printer can constitute a personal area network.

Beyond the personal area network we have longer-range networks which are broadly classified networks as

· LAN

· MAN

· WAN

We will see these three networks in detail later. Finally the connection of two or more networks is called an inter-network. The world wide Internet is a well known example of inter-network. Distance is important as a classification metric as different techniques are used at different scales.

Local Area Networks

Local Area Networks are generally called LANs. They are privately owned networks within a single building or campus of up to few kilometers in size. Most of LAN’s use Bus or ring topology for connection and is illustrated as shown in fig. 1.5. They are used to connect personal computers and workstations in company offices and factories to share resources and exchange information. Traditional LANs run at speeds of 10Mbps to 100Mbps, have low delay (microseconds and nanoseconds) and make very few errors. Newer LANs operate at 10Gbps. Various topologies are possible for broadcast LANs.

Metropolitan Area Networks

A Metropolitan Area Networks, referred as MANs covers a city. The best known example is cable television network available in many cities. Earlier these were used for TV reception only but with changes a two way internet service could be provided. A MAN might look something like the system shown in figure 1.6. In this system both television signals and internet being fed into centralized head end for distribution to people’s home.

Cable television is not the only MAN. Recent developments in high speed wireless internet access also resulted in MAN.

A wide area network is referred as WAN. WAN spans a large geographical area often a continent or country. WAN contains a collection of machines, traditionally called as hosts. As illustrated in figure 1.7, these hosts can be on LANs and are connected by a subnet or also called communication subnet. The hosts are owned by customers or are personal computers. The communication subnets are owned by a telephone company or internet service provider. The subnet carries the messages from hosts to hosts, just as telephone system carries words from speaker to listener. Each host is connected to a LAN on which a router is present. Sometimes a host may be connected directly to a router. The collection of communication lines and routers is called a communication subnet.

In most WANs, the network contains many transmission lines each connecting a pair of routers. As illustrated in figure 1.8, a packet is sent from one router to another via one or more intermediate routers. The packet is received at each intermediate router in its entirety. That is store the packet in full until the required output line is free, and then forwards it. A subnet that works according to this principle is called store and forward or packet switched subnet. Not all WANs are packet switched. A second possibility for a WAN is a satellite system. Satellite networks are inherently broadcast networks.

Wireless Networks

The Italian physicist Guglielmo Marconi in 1901, demonstrated a ship-to-shore telegraph, using Morse code. Morse code is a collection of binary digits called dots and dashes. Modern digital wireless systems have better performance, but the basic idea is the same. Wireless networks are divided into three main categories. These are discussed below.

System interconnection

It is all about interconnecting the components of a computer using short range radio. Every computer has a monitor, keyboard, mouse, printer connected to the main unit by cables. Bluetooth configuration is as shown in figure 1.9(a). Some companies got together to design a short range wireless network called Bluetooth to connect these components without wires.

Blue tooth allows digital cameras, headsets, scanners, and other devices like even computers to connect to a computer by merely being brought within range. No cables, no driver installation, just put them on and turn them on they work.

Wireless LANs

These are systems in which every computer has a radio modem and a antenna with which it can communicate with other systems. Often there is an antenna on the ceiling that the machines talk to as shown in figure 1.9(b). Wireless LANs are becoming common in small offices and homes, where installing Ethernet is considered too much trouble. Also used in

older buildings, company cafeterias, conference rooms etc. IEEE 802.11 is the standard for wireless LANs.

Wireless WANs

This is also wireless network but is a wide area system.

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

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:

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 protocols4. 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.

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.

Each layer of the ISO-OSI reference model are discussed below:

Physical Layer

This layer is the bottom most layer that is concerned with transmitting raw bits over the communication channel (physical medium). The design issues have to do with making sure that when one side sends a 1 bit, it is received by other side as a 1 bit, and not as a 0 bit. It performs direct transmission of logical information that is digital bit streams into physical phenomena in the form of electronic pulses. Modulators/demodulators are used at this layer. The design issue here largely deals with mechanical, electrical, and procedural interfaces, and the physical transmission medium, which lies below this physical layer.

In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices. The major functions and services performed by the physical layer are:· Establishment and termination of a connection to a communications medium. · Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. · Modulation, is a technique of conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and fiber optic) or over a radio link.

Parallel SCSI buses operate in this layer. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data-link layer. The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

2. Data Link Layer

The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. That is it makes sure that the message indeed reach the other end without corruption or without signal distortion and noise. It accomplishes this task by having the sender break the input data up into the frames called data frames. The DLL of transmitter, then transmits the frames sequentially, and processes acknowledgement frames sent back by the receiver. After processing acknowledgement frame, may be the transmitter needs to re-transmit a copy of the frame. So therefore the DLL at receiver is required to detect duplications of frames. The best known example of this is Ethernet. This layer manages the interaction of devices with a shared medium. Other examples of data link protocols are HDLC and ADCCP for point-to-

point or packet-switched networks and Aloha for local area networks. On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer. It arranges bits from the physical layer into logical chunks of data, known as frames.This is the layer at which the bridges and switches operate. Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding. Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain.The data link layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layers by protocols such as TCP.

3. Network Layer

The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer. The Network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme values are chosen by the network engineer. The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol (IP). Perhaps it’s easier to visualize this layer as managing the sequence of human carriers taking a letter from the sender to the local post office, trucks that carry sacks of mail to other post offices or airports, airplanes that carry airmail between major cities, trucks that distribute mail sacks in a city, and carriers that take a letter to its destinations. Think of fragmentation as splitting a large document into smaller envelopes for shipping, or, in the case of the network layer, splitting an application or transport record into packets.

The major tasks of network layer are listed

· It controls routes for individual message through the actual topology.

· Finds the best route.

· Finds alternate routes.

· It accomplishes buffering and deadlock handling.

4. Transport Layer

The Transport layer provides transparent transfer of data between end users, providing reliable data transfer while relieving the upper layers of it. The transport layer controls the reliability of a given link through flow control, segmentation/de-segmentation, and error control. Some protocols are state and connection oriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP). The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets. Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic Presentation services that can be read by the addressee only.

Roughly speaking, tunneling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM’s SNA or Novell’s IPX over an IP network, or end-to-end encryption with IP security (IP sec). While Generic Routing Encapsulation (GRE) might seem to be a network layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. The major tasks of Transport layer are listed below:· It locates the other party· It creates a transport pipe between both end-users.· It breaks the message into packets and reassembles them at the destination.· It applies flow control to the packet stream.

5. Session Layer

The Session layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for either full-duplex or half-duplex operation, and establishes check pointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session check pointing and recovery, which is not usually used in the Internet protocols suite.

The major tasks of session layer are listed

· It is responsible for the relation between two end-users.

· It maintains the integrity and controls the data exchanged between the end-users.

· The end-users are aware of each other when the relation is established (synchronization).

· It uses naming and addressing to identify a particular user.

· It makes sure that the lower layer guarantees delivering the message (flow control).

6. Presentation Layer

The Presentation layer transforms the data to provide a standard interface for the Application layer. MIME encoding, data encryption and similar manipulation of the presentation are done at this layer to present the data as a service or protocol developer sees fit. Examples of this layer are converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of XML.

The major tasks of presentation layer are listed below:

· It translates the language used by the application layer.

· It makes the users as independent as possible, and then they can concentrate on conversation.

7. Application Layer (end users)

The application layer is the seventh level of the seven-layer OSI model. It interfaces directly to the users and performs common application services for the application processes. It also issues requests to the presentation layer. Note carefully that this layer provides services to user-defined application processes, and not to the end user. For example, it defines a file transfer protocol, but the end user must go through an application process to invoke file transfer. The OSI model does not include human interfaces.

The common application services sub layer provides functional elements including the Remote Operations Service Element (comparable to Internet Remote Procedure Call), Association Control, and Transaction Processing (according to the ACID requirements). Above the common application service sub layer are functions meaningful to user application programs, such as messaging (X.400), directory (X.500), file transfer (FTAM), virtual terminal (VTAM), and batch job manipulation (JTAM).

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.

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 below

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

A Critique of the TCP/IP Reference Model

Problems:

· Service, interface, and protocol not distinguished

· Not a general model

· Host-to-network “layer” not really a layer

· No mention of physical and data link layers

· Minor protocols deeply entrenched, hard to replace

Network standardization

Network standardization is a definition that has been approved by a recognized standards organization. Standards exist for programming languages, operating systems, data formats, communications protocols, and electrical interfaces.

Two categories of standards:

· De facto (Latin for “from the fact”) standards:

These are those that have just happened without any formal plan. These are formats that have become standard simply because a large number of companies have agreed to use them. They have not been formally approved as standards E.g., IBM PC for small office computers, UNIX for operating systems in CS departments. PostScript is a good example of a de facto standard.

· De jure (Latin for “by law”) standards:

These are formal legal standards adopted by some authorized standardization body.

Two classes of standard organizations

· Organizations established by treaty among national governments.

· Voluntary, nontreaty organizations.

From a user’s standpoint, standards are extremely important in the computer industry because they allow the combination of products from different manufacturers to create a customized system. Without standards, only hardware and software from the same company could be used together. In addition, standard user interfaces can make it much easier to learn how to use new applications.

Most official computer standards are set by one of the following organizations:

· ANSI (American National Standards Institute)

· ITU (International Telecommunication Union)

· IEEE (Institute of Electrical and Electronic Engineers)

· ISO (International Standards Organization)

· VESA (Video Electronics Standards Association)

Benefits of standardization:

· Allow different computers to communicate.

· Increase the market for products adhering to the standard.

Who’s who in the telecommunication world?

Common carriers: private telephone companies (e.g., AT&T, USA).

· PTT (Post, Telegraph & Telephone) administration: nationalized telecommunication companies (most of the world).

· ITU (International Telecommunication Union): an agency of the UN for international telecommunication coordination.

· CCITT (an acronym for its French name): one of the organs of ITU (i.e., ITU-T), specialized for

telephone and data communication systems.

3. Explain the following with respect to Data Communications:

A) Fourier analysis B) Band limited signals

C) Maximum data rate of a channelAns –

A) Fourier analysis

In 19th century, the French mathematician Fourier proved that any periodic function of time g (t) with period T can be constructed by summing a number of cosines and sines.

(3.1)

Where f=1/T is the fundamental frequency, and are the sine and cosine amplitudes of the nth harmonics. Such decomposition is called a Fourier series.

B) Band limited signals

Consider the signal given in figure below. Figure shows the signal that is the ASCII representation of the character ‘b’ which consists of the bit pattern ‘01100010’ along with its harmonics.

Any transmission facility cannot pass all the harmonics and hence few of the harmonics are diminished and distorted. The bandwidth is restricted to low frequencies consisting of 1, 2, 4, and 8 harmonics and then transmitted. Figure 3.1(b) to 3.1(e) shows the spectra and reconstructed functions for these band-limited signals.

Limiting the bandwidth limits the data rate even for perfect channels. However complex coding schemes that use several voltage levels do exist and can achieve higher data rates.

C) Maximum data rate of a channel

In 1924, H. Nyquist realized the existence of the fundamental limit and derived the equation expressing the maximum data for a finite bandwidth noiseless channel. In 1948, Claude Shannon carried Nyquist work further and extended it to the case of a channel subject to random noise.

In communications, it is not really the amount of noise that concerns us, but rather the amount of noise compared to the level of the desired signal. That is, it is the ratio of signal to noise power that is important, rather than the noise power alone. This Signal-to-Noise Ratio (SNR), usually expressed in decibel (dB), is one of the most important specifications of any communication system. The decibel is a logarithmic unit used for comparisons of power levels

or voltage levels. In order to understand the implication of dB, it is important to know that a sound level of zero dB corresponds to the threshold of hearing, which is the smallest sound that can be heard. A normal speech conversation would measure about 60 dB.

If an arbitrary signal is passed through the Low pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H samples per second. Sampling the line faster than 2H per second is pointless. If the signal consists of V discrete levels, then Nyquist theorem states that, for a noiseless channel

Maximum data rate = 2H.log2 (V) bits per second. (3.2)

For a noisy channel with bandwidth is again H, knowing signal to noise ratio S/N, the maximum data rate according to Shannon is given as

Maximum data rate = H.log2 (1+S/N) bits per second. (3.3)

4. Explain the following concepts of Internetworking:

A) Internet architecture

B) Protocols and Significance for internetworking

C) Internet layering modelAns –

A) Internet Architecture: B1-226, B2-56

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.

B) Protocols and Significance for Internetworking

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.

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

C) 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.

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.

5. Explain the following different classes of IP addresses:

A) Primary classful addresses B) Class A

C) Class B D) Class CAns –

A) 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.

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.

B) 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.

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.

C) 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.

D) 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.

6. Discuss the following with suitable examples:

A) Variable length subnets B) Subnet MasksAns –

A) Variable Length Subnets

Most sites that implement subnetting use a fixed length assignment. The TCP/IP subnet standard provides even more flexibility than the fixed length subnetting seen above. An organization may select a subnet partition independently for each physical network. Although the technique is known as variable length subnetting, the name is slightly misleading as the value does not vary over time. Once a partition is been selected for a particular network the partition never changes. All hosts and routers attached to that network must follow the decision.

B) Subnet Masks

The subnet technology makes configuration of either fixed or variable length easy with the use of subnet masks. Usually subnetting is used by class B networks. The standard specifies a 32-bit subnet mask to identify the division. Thus a site using subnet addressing must choose a 32 bit subnet mask for each network.