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apter, h, C, , 10, , Computer Networks, , “Hoaxes use weaknesses in human, behavior to ensure they are replicated and, distributed. In other words, hoaxes prey on, the Human Operating System.”, — Stewart Kirkpatrick, In this Chapter, »» Introduction to Computer, Networks, »» Evolution of Networking, »» Types of Networks, »» Network Devices, »» Networking Topologies, »» Identifying Nodes, in a Networked, Communication, , 10.1 Introduction, , to, , Computer Networks, , We are living in a connected world. Information, is being produced, exchanged, and traced across, the globe in real time. It's possible as almost, everyone and everything in the digital world is, interconnected through one way or the other., , »» Internet, Web and the, Internet of Things, »» Domain Name System, , Figure 10.1: Interconnection forming a social network, 2021–22, , Chpater-10.indd 181, , 11-09-2020 16:41:16

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Activity 10.1, Identify some other, networks in the, real world., , A group of two or more similar things or people, interconnected with each other is called network (Figure, 10.1). Some of the examples of network in our everyday, life includes:, • Social network, • Mobile network, • Network of computers, • Airlines, railway, banks, hospitals networks, A computer network (Figure 10.2) is an interconnection, among two or more computers or computing devices., Such interconnection allows computers to share data, and resources among each other. A basic network may, connect a few computers placed in a room., The network size may vary from small to large, depending on the number of computers it connects., A computer network can include different types of, hosts (also called nodes) like server, desktop, laptop,, cellular phones., , Networking, Device, , Figure 10.2: A computer network, , Apart from computers, networks include networking, devices like switch, router, modem, etc. Networking, devices are used to connect multiple computers in, different settings. For communication, data in a network, is divided into smaller chunks called packets. These, packets are then carried over a network. Devices in a, network can be connected either through wired media, like cables or wireless media like air., In a communication network, each device that is a, part of a network and that can receive, create, store, or send data to different network routes is called a, node. In the context of data communication, a node, can be a device such as a modem, hub, bridge, switch,, router, digital telephone handset, a printer, a computer, or a server., , Computer Science - Class XII, , 182, 2021–22, , Chpater-10.indd 182, , 11-09-2020 16:41:17

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Interconnectivity of computing devices in a network, allows us to exchange information simultaneously with, many parties through email, websites, audio/video, calls, etc. Network allows sharing of resources. For, example, a printer can be made available to multiple, computers through a network; a networked storage, can be accessed by multiple computers. People often, connect their devices through hotspot, thus forming a, small personal network., , 10.2 Evolution, , of, , Activity 10.2, Create a hotspot using, a smartphone and, connect other, devices to it., , Networking, , In the 1960s a research project was commissioned by, Advanced Research Projects Agency Network (ARPANET), in the U.S. Department of Defence to connect the, academic and research institutions located at different, places for scientific collaborations. The first message was, communicated between the University of California, Los, Angeles (UCLA) and Stanford Research Institute (SRI)., Slowly but gradually, more and more organisations, joined the ARPANET, and many independent smaller, networks were formed. Few of the milestones in the, magnificent journey of evolution of computer networks, is depicted in the timeline shown in Figure 10.3., The idea of Advanced, Research Project Agency, Network (ARPANET) is, conceptualized, , 1961, , First version of Wi-fi, (802.11) standard was, introduced, , TCP/IP introduced as, standard protocol on, ARPANET, , National Science, Foundation brings, connectivity to more, people with its NSFNET, program, , Roy Tomlinson develops, network messaging or, E-mail. Symbol @, comes to mean "at", 1982, 1971, , 1986, , 1969, , ARPANET became, functional by, connecting UCLA and, SRI, , 1997, , 1983, 1974, , Domain Name System, introduced, , The term Internet was, coined,, First commercial use of, ARPANET, was started, in the name of Telenet, , 1990, , The Berners-Lee at, CERN developed HTML, and URL, thus giving, birth to World Wide Web, (www), , Figure 10.3: Timeline showing evolution of networking, , Computer Networks, , 183, 2021–22, , Chpater-10.indd 183, , 11-09-2020 16:41:17

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Notes, , 10.3 Types, , of, , Networks, , There are various types of computer networks ranging, from network of handheld devices (like mobile phones, or tablets) connected through Wi-Fi or Bluetooth within, a single room to the millions of computers spread across, the globe. Some are connected wireless while others are, connected through wires., Based on the geographical area covered and data, transfer rate, computer networks are broadly categorised, as:, • PAN ( Personal Area Network), • LAN (Local Area Network), • MAN (Metropolitan Area Network), • WAN (Wide Area Network), 10.3.1 Personal Area Network (PAN), It is a network formed by connecting a few personal, devices like computers, laptops, mobile phones, smart, phones, printers etc., as shown in Figure 10.4. All these, devices lie within an approximate range of 10 metres., A personal area network may be wired or wireless., For example, a mobile phone connected to the laptop, through USB forms a wired PAN while two smartphones, communicating with each other through Bluetooth, technology form a wireless PAN or WPAN., , Figure 10.4:, , A Personal Area Network, , Computer Science - Class XII, , 184, 2021–22, , Chpater-10.indd 184, , 11-09-2020 16:41:17

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10.3.2 Local Area Network (LAN), It is a network that connects computers, mobile phones,, tablet, mouse, printer, etc., placed at a limited distance., The geographical area covered by a LAN can range from, a single room, a floor, an office having one or more, buildings in the same premise, laboratory, a school,, college, or university campus. The connectivity is done, by means of wires, Ethernet cables, fibre optics, or Wi-Fi., A Local Area Network (LAN) is shown in Figure 10.5., , Figure 10.5:, , Explore and find out, the minimum internet, speed required to, make a video call., , A Local Area Network, , LAN is comparatively secure as only authentic, users in the network can access other computers or, shared resources. Users can print documents using, a connected printer, upload/download documents, and software to and from the local server. Such LANs, provide the short range communication with the high, speed data transfer rates. These types of networks can, be extended up to 1 km. Data transfer in LAN is quite, high, and usually varies from 10 Mbps (called Ethernet), to 1000 Mbps (called Gigabit Ethernet), where Mbps, stands for Megabits per second. Ethernet is a set of rules, that decides how computers and other devices connect, with each other through cables in a local area network, or LAN., 10.3.3 Metropolitan Area Network (MAN), Metropolitan Area Network (MAN) is an extended form of, LAN which covers a larger geographical area like a city or, a town. Data transfer rate in MAN also ranges in Mbps,, Computer Networks, , 185, 2021–22, , Chpater-10.indd 185, , 11-09-2020 16:41:18

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but it is considerably less as compared to LAN. Cable TV, network or cable based broadband internet services are, examples of MAN. This kind of network can be extended, up to 30-40 km. Sometimes, many LANs are connected, together to form MAN, as shown in Figure 10.6., , LAN 1, , Networking, Device, , LAN 3, , LAN 2, , Figure 10.6: A Metropolitan Area Network, , It is possible to access, your bank account, from any part of the, world. Whether the, bank’s network is a, LAN, MAN, WAN or, any other type?, , 10.3.4 Wide Area Network (WAN), Wide Area Network connects computers and other, LANs and MANs, which are spread across different, geographical locations of a country or in different, countries or continents. A WAN could be formed, by connecting a LAN to other LANs (Figure 10.7) via, wired/wireless media. Large business, educational, and government organisations connect their different, branches in different locations across the world through, WAN. The Internet is the largest WAN that connects, billions of computers, smartphones and millions of, LANs from different continents., , Computer Science - Class XII, , 186, 2021–22, , Chpater-10.indd 186, , 11-09-2020 16:41:18

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Network User, , Network User, , Network User, , Network User, , Network Switch, , Network User, , Network Switch, , Network User, , Internet, Network User, , Network User, , Network User, , LAN 1 - Delhi, , Network User, , LAN 1 - Shimla, , Figure 10.7: A Wide Area Network, , 10.4 Network Devices, To communicate data through different transmission, media and to configure networks with different, functionality, we require different devices like Modem,, Hub, Switch, Repeater, Router, Gateway, etc. Let us, explore them in detail., 10.4.1 Modem, Modem stands for ‘MOdulator DEModulator’. It refers to, a device used for conversion between analog signals and, digital bits. We know computers store and process data, in terms of 0s and 1s. However, to transmit data from, a sender to a receiver, or while browsing the internet,, digital data are converted to an analog signal and the, medium (be it free-space or a physical media) carries, the signal to the receiver. There are modems connected, to both the source and destination nodes. The modem, at the sender’s end acts as a modulator that converts, the digital data into analog signals. The modem at the, receiver’s end acts as a demodulator that converts, the analog signals into digital data for the destination, node to understand. Figure 10.8 shows connectivity, using a modem., , Computer Networks, , 187, 2021–22, , Chpater-10.indd 187, , 11-09-2020 16:41:19

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Analog Signal, , Modulation, Digital Signal, , Demodulation, , Demodulation, , Modulation, , Digital Signal, , Telephone Line, Modem, , Modem, , Figure 10.8: Use of modem, , Figure 10.9:, , 10.4.2 Ethernet Card, Ethernet card, also known as Network Interface Card, (NIC card in short) is a network adapter used to set, up a wired network., It acts as an interface, between computer and, the network. It is a circuit, board mounted on the, motherboard of a computer, as, shown, in, Figure, 10.9. The Ethernet cable, connects the computer to, the network through NIC., Ethernet cards can support, data transfer between 10, Mbps and 1 Gbps (1000, Mbps). Each NIC has a, MAC address, which helps, in uniquely identifying the, computer on the network., , A Network Interface Card, , 10.4.3 RJ45, RJ 45 or Registered Jack-45 is an, eight-pin connector (Figure 10.10), that is used exclusively with, Ethernet cables for networking., It is a standard networking, interface that can be seen at, the end of all network cables., Basically, it is a small plastic plug, that fits into RJ-45 jacks of the, Ethernet cards present in various, computing devices., , Figure 10.10: RJ 45, , Computer Science - Class XII, , 188, 2021–22, , Chpater-10.indd 188, , 11-09-2020 16:41:19

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10.4.4 Repeater, Data are carried in the form of signals over the cable., These signals can travel a specified distance (usually, about 100 m). Signals lose their strength beyond this, limit and become weak. In such conditions, original, signals need to be regenerated., A repeater is an analog device that works with signals, on the cables to which it is connected. The weakened, signal appearing on the cable is regenerated and put, back on the cable by a repeater., , An Internet service, provider (ISP) is any, organisation that, provides services, for accessing the, Internet., , 10.4.5 Hub, An Ethernet hub (Figure 10.11) is a network device used, to connect different devices through wires. Data arriving, on any of the lines are sent out on all the others. The, limitation of Hub is that if data from two devices come, at the same time, they will collide., Activity 10.3, , 1, , 2, , 3, , 4, , 5, , 6, , 7, , 8, , Find and list a, few ISPs in your, region., , Figure 10.11: A network hub with 8 ports, , 10.4.5 Switch, A switch is a networking device (Figure 10.12) that, plays a central role in a Local Area Network (LAN). Like, a hub, a network switch is used to connect multiple, computers or communicating devices. When data, arrives, the switch extracts the, destination address from the data, packet and looks it up in a table to, see where to send the packet. Thus,, it sends signals to only selected, devices instead of sending to all., It can forward multiple packets at, the same time. A switch does not, forward the signals which are noisy, or corrupted. It drops such signals, Figure 10.12: Cables connected to a network switch, and asks the sender to resend it., Ethernet switches are common in homes/offices, to connect multiple devices thus creating LANs or to, access the Internet., , Computer Networks, , 189, 2021–22, , Chpater-10.indd 189, , 11-09-2020 16:41:20

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Notes, , 10.4.6 Router, A router (Figure 10.13) is a network device that can, receive the data, analyse it and transmit it to other, networks. A router connects a local area network to the, internet. Compared to a hub or a switch, a router has, advanced capabilities as it can analyse the data being, carried over a network, decide/alter how it is packaged,, and send it to another network of a different type. For, example, data has been divided into packets of a certain, size. Suppose these packets are to be carried over a, different type of network which cannot handle bigger, packets. In such a case, the data is to be repackaged, as smaller packets and then sent over the network by, a router., , Figure 10.13: A router, , A router can be wired or wireless. A wireless router, can provide Wi-Fi access to smartphones and other, devices. Usually, such routers also contain some ports, to provide wired Internet access. These days, home Wi-Fi, routers perform the dual task of a router and a modem/, switch. These routers connect to incoming broadband, lines, from ISP (Internet Service Provider), and convert, them to digital data for computing devices to process., 10.4.7 Gateway, As the term “Gateway” suggests, it is a key access point, that acts as a “gate” between an organisation's network, and the outside world of the Internet (Figure 10.14)., Gateway serves as the entry and exit point of a network,, as all data coming in or going out of a network must, first pass through the gateway in order to use routing, paths. Besides routing data packets, gateways also, maintain information about the host network's internal, connection paths and the identified paths of other, remote networks. If a node from one network wants to, communicate with a node of a foreign network, it will, Computer Science - Class XII, , 190, 2021–22, , Chpater-10.indd 190, , 11-09-2020 16:41:20

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pass the data packet to the gateway, which then routes, it to the destination using the best possible route., , 10.0.0.0/8, IP ADDRESS, , Server, , PC 4, , PC 5, , PC 1, , Server, , GATEWAY, , PC 2, , 20.0.0.0/8, IP ADDRESS, , PC 4, , PC 3, , PC 1, , PC 5, , PC 2, , PC 3, , Figure 10.14: A network gateway, , For simple Internet connectivity at homes, the, gateway is usually the Internet Service Provider that, provides access to the entire Internet. Generally, a, router is configured to work as a gateway device, in computer networks. But a gateway can be, implemented completely in software, hardware, or, a combination of both. Because a network gateway, is placed at the edge of a network, the firewall is, usually integrated with it., , 10.5 Networking Topologies, We have already discussed that a number of computing, devices are connected together to form a Local Area, Network (LAN), and interconnections among millions of, LANs forms the Internet. The arrangement of computers, and other peripherals in a network is called its topology., Common network topologies are Mesh, Ring, Bus, Star, and Tree., , Computer Networks, , 191, 2021–22, , Chpater-10.indd 191, , 11-09-2020 16:41:21

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10.5.1 Mesh Topology, In this networking topology, each communicating, device is connected with every other device in the, network as shown in Figure 10.15. Such a network can, handle large amounts of traffic since multiple nodes, can transmit data simultaneously. Also, such networks, are more reliable in the sense that even if a node gets, down, it does not cause any break in the transmission, of data between other nodes. This topology is also, more secure as compared to other topologies because, each cable between two nodes carries different data., However, wiring is complex and cabling cost is high in, creating such networks and there are many redundant, or unutilised connections., , Figure 10.15: A mesh topology, To build a fullyconnected mesh, topology of n nodes,, it requires n(n-1)/2, wires., , 10.5.2 Ring Topology, In ring topology (Figure 10.16), each node is connected, to two other devices, one each on either side, as shown, in Figure 10.16. The nodes connected with each, other thus forms a ring. The link in a ring topology is, unidirectional. Thus, data can be transmitted in one, direction only (clockwise or counterclockwise)., , Figure 10.16: A ring topology, , 10.5.3 Bus Topology, In bus topology (Figure 10.17), each communicating, device connects to a transmission medium, known as, bus. Data sent from a node are passed on to the bus, and hence are transmitted to the length of the bus in, both directions. That means, data can be received by, any of the nodes connected to the bus., Computer Science - Class XII, , 192, 2021–22, , Chpater-10.indd 192, , 11-09-2020 16:41:21

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Bus, , Figure 10.17: A bus topology, , In this topology, a single backbone wire called bus is, shared among the nodes, which makes it cheaper and, easier to maintain. Both ring and bus topologies are, considered to be less secure and less reliable., 10.5.4 Star Topology, In star topology (Figure 10.18), each communicating, device is connected to a central node, which is a, networking device like a hub or a switch, as shown in, Figure 10.18., , How will a Bus and, Ring topology behave, in case a Node is, down?, , Star topology is considered very effective, efficient, and fast as each device is directly connected with the, central device. Although disturbance in one device will, not affect the rest of the network, any failure in a central, networking device may lead to the failure of complete, network., , Figure 10.18: A star topology, , The central node can be either a broadcasting device, means data will be transmitted to all the nodes in the, network, or a unicast device means the node can identify, the destination and forward data to that node only., 10.5.5 Tree or Hybrid Topology, It is a hierarchical topology, in which there are multiple, branches and each branch can have one or more basic, topologies like star, ring and bus. Such topologies are, usually realised in WANs where multiple LANs are, connected. Those LANs may be in the form of a ring,, bus or star. In figure 10.19, a hybrid topology is shown, connecting 4-star topologies in a bus., In this type of network, data transmitted from source, first reaches the centralised device and from there the, data passes through every branch where each branch, can have links for more nodes., Computer Networks, , 193, 2021–22, , Chpater-10.indd 193, , 11-09-2020 16:41:21

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Figure 10.19: A hybrid topology, , 10.6 Identifying Nodes, Communication, , in a, , Networked, , Each node in a network should be uniquely identified, so that a network device can identify the sender and, receiver and decide a routing path to transmit data., Let us explore further and know how each node is, distinguished in a network., 10.6.1 MAC Address, MAC stands for Media Access Control. The MAC address,, also known as the physical or hardware address, is a, unique value associated with a network adapter called, a NIC. The MAC address is engraved on NIC at the time, of manufacturing and thus it is a permanent address, and cannot be changed under any circumstances. The, machine on which the NIC is attached, can be physically, identified on the network using its MAC address., Each MAC address is a 12-digit hexadecimal numbers, (48 bits in length), of which the first six digits (24 bits), contain the manufacturer’s ID called Organisational, Unique Identifier (OUI) and the later six digits (24 bits), represents the serial number assigned to the card by, the manufacturer. A sample MAC address looks like:, , Activity 10.4, Explore how can you, find the MAC, address of your, computer, system., , 10.6.2 IP Address, IP address, also known as Internet Protocol address,, is also a unique address that can be used to uniquely, identify each node in a network. The IP addresses, Computer Science - Class XII, , 194, 2021–22, , Chpater-10.indd 194, , 11-09-2020 16:41:21

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are assigned to each node in a network that uses the, Internet Protocol for communication. Thus, if we know, a computer’s IP address, we can communicate with, that computer from anywhere in the world. However,, unlike MAC address, IP address can change if a node, is removed from one network and connected to another, network., The initial IP Address called version 4 (IPV4 in short),, is a 32 bit numeric address, written as four numbers, separated by periods, where each number is the decimal, (base-10) representation for an 8-bit binary (base-2), number and each can take any value from 0 - 255. A, sample IPV4 address looks like:, , Do mobile phones, have a MAC address?, Is it different from, the IMEI number of, mobile phones?, , 192:168:0:178, With more and more devices getting connected to, the Internet, it was realised that the 32-bit IP address, will not be sufficient as it offers just under 4.3 billion, unique addresses. Thus, a 128 bits IP address, called IP, version 6 (IPV6 in short) was proposed. An IPv6 address, is represented by eight groups of hexadecimal (base-16), numbers separated by colons. A sample IPV6 address, looks like:, , 2001:CDBA:0000:0000:0000:0000:3257:9652, 10.7 Internet, Web, , and the, , Internet, , of, , Things, , The Internet is the global network of computing devices, including desktop, laptop, servers, tablets, mobile, phones, other handheld devices, printers, scanners,, routers, switches, gateways, etc. Moreover, smart, electronic appliances like TV, AC, refrigerator, fan, light,, etc. can also communicate through a network. The list, of such smart devices is always increasing e.g., drones,, vehicles, door lock, security camera. We have already, studied IoT and WoT in class 11., The Internet is evolving every day and it is difficult, to visualise or describe each and every aspect of the, architecture of the Internet. Computers are either, connected to a modem through a cable or wirelessly (WiFi). That modem, be it wired or wireless, is connected to, a local Internet Service Provider (ISP) who then connects, to a national network. Many such ISPs connect together, forming a regional network and regional networks, connect together forming a national network, and such, country-wise networks form the Internet backbone., Computer Networks, , 195, 2021–22, , Chpater-10.indd 195, , 11-09-2020 16:41:21

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You are encouraged, to take up any area, of concern where, you think IoT can be, immensely beneficial, and discuss it with, your peers. An, example for the same, can be preventing, road accidents., , The Internet today is a widespread network, and its, influence is no longer limited to the technical fields of, computer communications. It is being used by everyone, in the society as is evident from the increasing use of, online tools for education, creativity, entertainment,, socialisation, and e-commerce., 10.7.1 The World Wide Web (WWW), The World Wide Web (WWW) or web in short, is an, ocean of information, stored in the form of trillions, of interlinked web pages and web resources. The, resources on the web can be shared or accessed, through the Internet., Earlier, to access files residing in different, computers, one had to login individually to each, computer through the Internet. Besides, files in, different computers were sometimes in different, formats, and it was difficult to understand each other’s, files and documents. Sir Tim Berners-Lee — a British, computer scientist invented the revolutionary World, Wide Web in 1990 by defining three fundamental, technologies that lead to creation of web:, •, , •, , HTML – HyperText Markup Language. It is a language, which is used to design standardised Web Pages so, that the Web contents can be read and understood, from any computer. Basic structure of every webpage, is designed using HTML., URI – Uniform Resource Identifier. It is a unique, address or path for each resource located on the, web. It is also known as Uniform Resource Locator, (URL). Every page on the web has a unique URL., Examples are: https://www.mhrd.gov.in,http://, www.ncert.nic.in,http://www.airindia.in, etc. URL, is sometimes also called web address. However,, a URL is not only the domain name. It contains, other information that completes a web address,, as depicted below:, Domain Name, , http://www.ncert.nic.in/textbook/textbook.htm, URL, , •, , HTTP – The HyperText Transfer Protocol is a set of, rules which is used to retrieve linked web pages, across the web. The more secure and advanced, version is HTTPS., Computer Science - Class XII, , 196, 2021–22, , Chpater-10.indd 196, , 11-09-2020 16:41:21

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Many people confuse the web with the Internet., The Internet as we know is the huge global network, of interconnected computers, which may or may not, have any file or webpage to share with the world. The, web on the other hand is the interlinking of collection, of Webpages on these computers which are accessible, over the Internet. WWW today gives users access to a, vast collection of information created and shared by, people across the world. It is today the most popular, information retrieval system, , Notes, , 10.8 Domain Name System, , The Internet is a vast ocean where information is, available in the form of millions of websites. Each website, is stored on a server which is connected to the Internet,, which means each server has an IP address. Every, device connected to the Internet has an IP address. To, access a website, we need to enter its IP address on our, web browser. But it is very difficult to remember the IP, addresses of different websites as they are in terms of, numbers or strings., However, it is easier to remember names, and, therefore, each computer server hosting a website or, web resource is given a name against its IP address., These names are called the Domain names or hostnames, corresponding to unique IP addresses assigned to each, server. For easy understanding, it can be considered, as the phonebook where instead of remembering each, person’s phone number, we assign names to their, numbers. For example, IP addresses and domain names, of some websites are as follows:, Table 10.1 Examples of domain names and their, mapped IP addresses, Domain Name, , IP Address, , ncert.nic.in, , 164.100.60.233, , cbse.nic.in, , 164.100.107.32, , mhrd.gov.in, , 164.100.163.45, , wikipedia.org, , 198.35.26.96, , 10.8.1 DNS Server, Instead of remembering IP addresses, we assign a, domain name to each IP. But, to access a web resource,, a browser needs to find out the IP address corresponding, to the domain name entered. Conversion of the domain, Computer Networks, , 197, 2021–22, , Chpater-10.indd 197, , 11-09-2020 16:41:21

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name of each web server to its corresponding IP address, is called domain name resolution. It is done through, a server called DNS server. Thus, when we enter a, URL on a web browser, the HTTP protocol approaches, a computer server called DNS server to obtain the IP, address corresponding to that domain name. After, getting the IP address, the HTTP protocol retrieves the, information and loads it in our browser., In Figure 10.20, an example is shown in which the, HTTP requests a DNS server for corresponding IP addss,, and the server sends back an IP address., DNS root servers, are named using, alphabets A through, M for the first, 13 letters of the, alphabet. Ten of, these servers are, in the US, one in, London, one in, Stockholm, and, one in Japan., The organisation, Internet Assigned, Numbers Authority, (IANA) keeps this, list of DNS root, servers., , User, , 164.100.60.233, HTTP in, Browser, , www.ncert.nic.in, , DNS, Server, , Figure 10.20: Request of IP address corresponding to domain name, , A DNS server maintains a database of domain names, and their corresponding IP addresses. To understand, how the domain name resolution works, we have to, understand how and where the DNS servers are kept., The DNS servers are placed in hierarchical order. At, the top level, there are 13 servers called root servers., Then below the root servers there are other DNS servers, at different levels. A DNS server may contain the IP, address corresponding to a domain or it will contain, the IP address of other DNS servers, where this domain, entry can be searched., , Summary, •, •, •, , A computer network is an interconnection among, two or more computers or computing devices., A computer network allows computers to share, data and resources among each other., Networking devices are used to connect multiple, computers in different settings., , Computer Science - Class XII, , 198, 2021–22, , Chpater-10.indd 198, , 11-09-2020 16:41:21

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•, , •, , •, , •, •, •, •, , •, •, •, •, •, •, •, , In a communication network, each device that is, a part of a network and that can receive, create,, store or send data to different network routes is, called a node., Based on the geographical area covered and data, transfer rate, computer networks are broadly, categorised into LAN (Local Area Network), MAN, (Metropolitan Area Network) and WAN (Wide Area, Network)., LAN is a network that connects a variety of nodes, placed at a limited distance ranging from a single, room, a floor, an office or a campus having one or, more buildings in the same premises., Ethernet is a set of rules that decides how, computers and other devices connect with each, other through cables in a LAN., Metropolitan Area Network (MAN) is an extended, form of LAN which covers a larger geographical, area like a city or a town., Cable TV network or cable based broadband, internet services are examples of MAN., Wide Area Network (WAN) connects computers, and other LANs and MANs, which are spread, across different geographical locations of a, country or in different countries or continents., The Internet is the largest WAN that connects, billions of computers, smartphones and millions, of LANs from different continents., Modem stands for ‘MOdulator DEModulator’,, is a device used for conversion between electric, signals and digital bits., Ethernet card, also known as Network Interface, Card (NIC card in short) is a network adaptor, used to set up a wired network., Each NIC has a MAC address, which helps in, uniquely identifying the computer on the network., A repeater is an analog device that regenerate the, signals on the cables to which it is connected., A switch is a networking device used to connect, multiple computers or communicating devices., A router is a network device that can receive the, data, analyse it and transmit it to other networks., , Computer Networks, , Notes, , 199, 2021–22, , Chpater-10.indd 199, , 11-09-2020 16:41:21

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Notes, , •, , •, •, •, •, •, •, •, •, , •, •, •, •, •, •, , Gateway serves as the entry and exit point of a, network, as all data coming in or going out of a, network must first pass through the gateway in, order to use routing paths., The arrangement of computers and other, peripherals in a network is called its topology., Common network topologies are Mesh, Ring, Bus,, Star and Tree., In mesh topology each communicating device is, connected with every other device in the network., In ring topology, each node is connected to two, other devices, one each on either side., In bus topology, a single backbone wire called, bus is shared among the nodes, which makes it, cheaper and easy to maintain., In star topology, each communicating device is, connected to a central networking device like a, hub or a switch., In tree or hybrid topology, there are multiple, branches and each branch can have one or more, basic topologies like star, ring and bus., The MAC address, also known as the physical or, hardware address, is a unique permanent value, associated with a network adapter called a NIC., It is used to physically identify a machine on the, network., IP address, also known as Internet Protocol, address, is a unique address that can be used to, uniquely identify each node in a network., Unlike MAC address, IP address can change if a, node is removed from one network and connected, to another network., The Internet is the global network of computing, devices., The World Wide Web (WWW) or web in short, is an, ocean of information, stored in the form of trillions, of interlinked web pages and web resources., Sir Tim Berners-Lee — a British computer, scientist invented the revolutionary World Wide, Web in 1990., HTML (HyperText Markup Language) is a, language which is used to design standardised, Web Pages so that the Web contents can be read, Computer Science - Class XII, , 200, 2021–22, , Chpater-10.indd 200, , 11-09-2020 16:41:21

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•, •, , •, , •, , and understood from any computer., URI (Uniform Resource Identifier) or URL (Uniform, Resource Locator) is a unique address or path for, each resource located on the web., HTTP – The HyperText Transfer Protocol is a set of, rules which is used to retrieve linked web pages, across the web. The more secure and advanced, version is HTTPS., Each computer server hosting a website or web, resource is given a name against its IP address., These names are called the Domain names or, hostnames., Conversion of the domain name of each web server, to its corresponding IP address is called domain, name resolution. It is done through a server called, DNS server., , Notes, , Exercise, 1. Expand the following:, a) ARPANET, b) MAC, c) ISP, d) URI, 2. What do you understand by the term network?, 3. Mention any two main advantages of using a network of, computing devices., 4. Differentiate between LAN and WAN., 5. Write down the names of few commonly used networking, devices., 6. Two universities in different States want to transfer, information. Which type of network they need to use for, this?, 7. Define the term topology. What are the popular network, topologies?, 8. How is tree topology different from bus topology?, 9. Identify the type of topology from the following:, a) Each node is connected with the help of a single cable., b) Each node is connected with central switching, through independent cables., , Computer Networks, , 201, 2021–22, , Chpater-10.indd 201, , 11-09-2020 16:41:21

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Notes, , 10. What do you mean by a modem? Why is it used?, 11. Explain the following devices:, a) Switch, b) Repeater, c) Router, d) Gateway, e) NIC, 12. Draw a network layout of star topology and bus, topology connecting five computers., 13. What is the significance of MAC address?, 14. How is IP address different from MAC address?, Discuss briefly., 15. What is DNS? What is a DNS server?, 16. Sahil, a class X student, has just started understanding, the basics of Internet and web technologies. He is a bit, confused in between the terms “World Wide Web” and, “Internet”. Help him in understanding both the terms, with the help of suitable examples of each., , Computer Science - Class XII, , 202, 2021–22, , Chpater-10.indd 202, , 11-09-2020 16:41:21