NCERT Solutions Class 12 Computer Science Chapter 10 Computer Networks

Tie each abbreviation to where it sits in the network story, because then the full forms stop being random letters and become a small map.

• ARPANET is the historical root of the Internet, the first packet-switched network, so any question on the origin of the Internet points back to this name. Spelling "Agency" rather than "Agencies" protects the mark.
• MAC sits on the hardware: every network card leaves the factory with a unique 48-bit MAC address, so it is the physical device address on the local network.
• ISP sits at the boundary between your home network and the wider Internet, and it is the body that hands your connection a public IP address.
• URI sits where humans type: it is the general name for any web resource, and saying "a URL is a kind of URI" shows you understand the relationship.

Write the expansion, then a four or five word note on what stage it belongs to, because that extra phrase both proves understanding and helps you recall the spelling under pressure.

Answer: ARPANET = Advanced Research Projects Agency Network; MAC = Media Access Control; ISP = Internet Service Provider; URI = Uniform Resource Identifier.

Define it, then justify with the purpose, because the purpose is half the marks. A strong definition says what a network is made of and why we build one.

• The structural half: two or more devices, called nodes, connected by a medium that may be wired (a cable) or wireless (Wi-Fi), so they can pass data.
• The often-forgotten half: the reason. Devices are networked to share resources and communicate, so name one or two shared resources, a printer, a file, an Internet connection.
• Mention protocols, because two devices can be physically wired and still not talk if they do not follow the same rules; TCP/IP is the usual example.

Watch one slip: a network needs at least two devices, so saying "two or more" stops you from calling a single stand-alone computer a network. Close with one everyday example, a school lab sharing one printer and one Internet line.

Answer: A network is two or more devices (nodes) linked by a wired or wireless medium and following shared protocols, built so the devices can communicate and share resources such as files, printers and an Internet connection.

Answer with a benefit and its one-line "because", and you cover the marks safely. The scheme awards a point for each clearly stated benefit, usually with a short reason.

• State resource sharing first, because it is the very purpose of a network: one printer, scanner, disk, software licence and Internet connection serve every machine, which saves money. Note the shared resource can be hardware, software or data.
• Pick fast communication and data sharing second: files and messages travel in seconds, so teams in different locations collaborate without moving storage around.
• If you have room, add reliability (copies on two machines) and centralised control as bonuses.

Do not confuse advantages of a network with features of the Internet specifically; keep examples device level, such as a shared office printer. Finish with a school lab where thirty computers share one printer and one broadband line.

Answer: The two strongest advantages are resource sharing (one printer, disk, software or Internet line used by all, which cuts cost) and fast communication and data sharing (files and messages move between devices in seconds), with reliability and central management as further gains.

Anchor every difference to the one root cause: distance. Almost every row in the table flows from how far the data must travel.

• Because a LAN's distances are short, signals stay strong and arrive quickly, so it is fast and low error; a WAN's long distances mean weaker, delayed signals, so it is slower and more error prone, which is why WANs lean on repeaters.
• Ownership follows too: a LAN sits in one site so one organisation owns it, whereas a WAN crosses public ground and many regions, so no single body owns it, which is why the Internet has no single owner.
• Cost and medium: a LAN uses cheap own cabling or Wi-Fi, while a WAN must use costly public links such as leased lines, fibre or satellite.

Present this as a table with a Basis column, list area, speed, cost, ownership and an example, and add that a MAN (Metropolitan Area Network) at city size sits between the two.

Answer: Distance is the root difference: a LAN covers a small area so it is fast, cheap, low error and owned by one organisation; a WAN covers a very large area so it is slower, costlier, higher error and spans many owners, with a MAN sitting between them at city size.

List the devices, but group them by the job they do so the list has shape and you do not forget one under pressure.

• Connect: hub (copies to every port) and switch (reads MAC, forwards only to the correct port, so faster and more secure, and has mostly replaced the hub).
• Extend: repeater (regenerates and boosts a weak signal so the network stretches further) and bridge (joins two segments of the same network).
• Interconnect: router (forwards between different networks using IP, best path) and gateway (connects different-protocol networks and translates).
• Access: modem (digital to analog and back over a phone or cable line) and NIC (physically attaches a device and holds its MAC address).

Presenting the answer in these four groups, connect, extend, interconnect and access, shows the examiner you understand what each device is for rather than having memorised a flat list.

Answer: Common networking devices are the modem, hub, switch, repeater, bridge, router, gateway and NIC; grouping them as connectors, extenders, interconnectors and access pieces makes the list easy to recall.

Decide by distance, then name the type and justify in one breath, because the marker wants both the right name and the reason tied to the distance.

• The deciding clue is "different States", which means hundreds of kilometres, far beyond one building or city, so it rules out PAN, LAN and MAN at once.
• What remains is the WAN, the category built to connect devices over very large distances, so a WAN is the answer and the reason is the inter-state distance.
• Avoid the trap of answering "Internet" as a separate type: the Internet is an example of a WAN, so the type is WAN and the Internet is how it is realised.

Keep a mental ladder of increasing size, PAN then LAN then MAN then WAN, and slot the distance onto the right rung; here "different States" lands firmly on WAN.

Answer: The correct type is a WAN, because the universities lie in different States, a range only a Wide Area Network covers; practically they would transfer the data over the Internet, which is the largest WAN, while PAN, LAN and MAN are all too small.

Give a sharp definition, then list the topologies with their one defining feature each, and attach the trade-off so the list sticks.

• Bus: all nodes hang off one shared backbone cable, so cheap on cable but a single backbone break halts the whole network.
• Star: every node has its own cable to a central hub or switch, so more cable but a broken cable knocks out only one node; the hub is the single critical point.
• Ring: nodes form a closed loop and data passes around the circle; a break can disrupt flow unless a dual ring is used.
• Tree: a hierarchy of star groups joined to a common backbone, suited to large layered networks. Mesh: every node links to every other, highest reliability at highest cost.

Adding the clause that the choice of topology affects cost, speed and failure behaviour lifts a bare definition into an understanding-level answer, and a quick labelled sketch of bus, star and ring often earns a diagram mark.

Answer: Topology is the arrangement of a network's nodes and links, and it shapes cost, speed and failure behaviour; the popular topologies are bus (one backbone), star (central hub), ring (closed loop), tree (hierarchy of stars) and mesh (every node to every node).

Build the contrast on structure first, then let cost and growth follow, because every other difference comes out of how the cable is laid.

• A bus is one single straight cable with every node tapped onto it, a flat line; a tree is an inverted tree, a root device, a common backbone, and several star groups hanging off it, so it has hierarchy and branching the bus does not.
• Saying plainly that a tree is a combination of star and bus topologies is a high-value line, because that is how the textbook frames it.
• Cost and growth follow: a bus uses the least cable and is cheapest but hard to extend, while a tree needs more cable plus a hub at each branch but is easy to extend, which suits large departmental networks.

Add a fault-behaviour sentence: a break in the single bus backbone can down the whole network, while a fault in one tree branch tends to affect only that branch. Present it as a table with a Basis column.

Answer: A bus is one flat straight backbone with all nodes on it, cheapest but hard to grow and vulnerable to a single break; a tree is a hierarchy of star groups on a common backbone (a star plus bus combination), costlier but easy to expand and more fault tolerant, suited to large layered networks.

Read the wiring phrase, match the keyword, name the topology, then add the one-line reason that earns the justification mark.

• In part (a) the deciding phrase is "a single cable" shared by each node, which is the exact definition of the backbone in a bus topology, so the answer is bus.
• In part (b) the phrases "central switching" and "independent cables" mean a central device with each node on its own cable, which is the signature of a star topology, so the answer is star.
• The word "central" is the strongest single hint for star.

A useful contrast to mention is failure behaviour: a break in the single bus backbone can halt the whole network, while a break in one star cable affects only that node, though a failure of the central switch would affect everyone.

Answer: a) Bus topology, because all nodes share one single backbone cable; b) Star topology, because each node has its own independent cable to a central switch or hub, which is the signature of a star.

Define from the name, then justify with the signal mismatch it fixes, because the name carries the whole idea.

• Modem is Modulator plus Demodulator, so saying this tells the examiner you know both halves of what the device does.
• The "why is it used" part is a mismatch of signal types: computers speak digital (0s and 1s), but telephone and many cable links carry analog (continuous waves), so without a translator the two cannot communicate over such a line.
• The conversion happens at both ends in opposite directions, so mention the pair of modems, one at each end, which shows you understand the full round trip.

A common slip is to confuse a modem with a router: a modem converts between digital and analog to reach the ISP's line, while a router shares one connection among many devices, so the two do different jobs even when a home box combines both.

Answer: A modem is a Modulator-Demodulator: at the sender it modulates digital data into an analog signal for the line, and at the receiver it demodulates the analog signal back into digital data. It is used because the line carries analog while the computer works in digital, so a translator is needed at each end.

Explain each device by its job and the address or signal it works on, and the differences become clear.

• Switch: forwards each packet only to the intended recipient by reading the destination MAC address, so it works with hardware addresses and beats the older hub.
• Repeater: makes no forwarding decisions; it simply regenerates and amplifies a weakened signal to stretch the network over distance.
• Router: works with IP addresses, consulting a routing table to choose the best path between different networks.
• Gateway: the one to handle with care, it connects two networks that speak different protocols and translates between them, so protocol translation is the keyword. NIC: physically attaches one device and carries its unique MAC address.

Present all five as a short table of device against its one job and the address or signal it handles, which stops you blurring the router and the gateway, the single most common error in this question.

Answer: Switch forwards by MAC to the correct port; repeater regenerates a weak signal to extend range; router forwards between different networks by IP on the best path; gateway joins networks of different protocols and translates; NIC physically connects a device and holds its MAC address.

Draw it correctly, then write the two or three features the drawing is meant to show, because the note secures marks even if the sketch is rough.

• Star: place a hub or switch in the middle and run one separate cable to each of the five computers; label the centre, because an unlabelled blob can be mistaken for a computer. Each computer has its own cable, so a break in one cable disconnects only that one, while a failure of the central hub brings the whole star down.
• Bus: draw one long straight backbone line and connect all five computers onto it with short drop links; label the line "backbone". All five share one cable, so it is cheap, but a break anywhere stops the whole network and only one computer can transmit at a time.
• Use exactly five computers numbered PC1 to PC5 in each, and keep the two diagrams clearly separate.

A real mark saver is to make the star truly radiate from the centre and the bus truly hang off one line; a star drawn as a loop or a bus drawn with a central box would lose the topology mark.

Answer: Star: a labelled central hub or switch with five separate cables, one to each of PC1 to PC5. Bus: one labelled straight backbone cable with PC1 to PC5 all tapped onto it (cheapest, but a backbone break stops all).

State what a MAC address is, then why it matters: uniqueness plus local delivery, because the question really asks "why does this address matter".

• Uniqueness: every NIC ever made gets its own MAC address, so on any network no two devices clash, which lets the network single out one exact machine, like a permanent serial number.
• Local delivery: within a local network a switch reads the destination MAC on each frame and forwards it only to the matching device, so data are not sprayed to everyone, making the network faster and more private than a hub.
• Permanence and teamwork: the MAC is fixed even when the IP changes; IP gets data to the right network and MAC gets it to the right device on that network.

Avoid the loose claim that a MAC address "locates a device on the Internet"; its real role is local identification and local delivery, so keep the significance tied to the local network.

Answer: A MAC address is the 48-bit, manufacturer-fixed, hardware address on a NIC. Its significance is twofold: it uniquely identifies every device so no two clash, and a switch uses it to deliver data to the exact device on the local network; being permanent, it complements the changeable IP address.

Contrast on several bases, but pin the whole difference on physical versus logical and local versus global.

• Physical versus logical: the MAC is built into the card and fixed for its life, while the IP is handed out by the network or ISP and changeable, so a laptop keeps its MAC everywhere but picks up a new IP on each network.
• Local versus global: the MAC identifies and delivers to a device within the local network, while the IP identifies a device across networks for worldwide routing.
• The remaining rows are easy marks: full forms, sizes (48-bit hex versus 32-bit IPv4 decimal), who assigns them, and an example of each.

The highest value sentence is how they cooperate: the IP carries your request across the Internet to the right network, then the MAC delivers it to the right device on that final network, which is why routers use IP and switches use MAC.

Answer: MAC is a fixed 48-bit physical address from the card maker for local delivery on a LAN; IP is a changeable 32-bit (IPv4) logical address from the network or ISP for global routing. They cooperate: IP routes data to the right network, then MAC delivers it to the right device.

Use the phone-book analogy, then separate the system from the server, because students often blur them.

• DNS is the phone book of the Internet: people remember names but computers connect using numeric IP addresses, and DNS translates the name you type into the IP number the machine needs.
• A DNS server is the actual computer that stores the table of names paired with IP addresses and answers lookup requests, so DNS is the overall system and a DNS server is one machine that runs it.
• DNS is hierarchical and distributed: no single server knows every name, so queries pass up a chain, from root servers to top-level domain servers like .com and then to the site's authoritative server, which is why it scales to the whole Internet.

A correctness point: DNS maps names to IP addresses as its main job, and it is about naming, not routing the data itself, so keep DNS as the lookup step that happens before routing begins.

Answer: DNS (Domain Name System) is the Internet's phone book: it translates a domain name into the numeric IP address the computer needs (name resolution). A DNS server is the computer that stores the name-to-IP records and answers these lookups, and the system is hierarchical so it scales across the whole Internet.

Explain it with the road-and-vehicle picture, then nail the part-whole relationship with examples.

• Picture the Internet as a vast system of roads, the cables, routers and links that connect computers worldwide, and the World Wide Web as one kind of vehicle that travels on those roads, the worldwide collection of linked web pages.
• The key idea for Sahil is the part-whole relationship: the Web is a part of the Internet, not the same as it, because the Internet also carries other services that are not the Web.
• Examples on both sides: opening www.wikipedia.org and clicking links is using the Web, while sending an email, making a video call or transferring a file uses the Internet but not the Web.

Add the touch that the Internet grew from ARPANET and is decades old, while the Web was invented later, in 1989 by Tim Berners-Lee, on top of the existing Internet; this ordering fixes which one contains the other. Steer Sahil away from treating "being online" and "being on the Web" as identical.

Answer: The Internet is the global infrastructure, the network of networks made of cables, routers and protocols; the World Wide Web is one service on it, the linked web pages opened in a browser. Opening www.wikipedia.org uses the Web; sending email uses the Internet but not the Web. The Web (invented 1989 by Tim Berners-Lee) is a part of the older Internet, not the whole of it.