Computer Networks

Written and reviewed by Rohit Singh, Computer Science & Commerce faculty at UniDrill (MCA, Osmania University · 6 years teaching). Based on NCERT Computer Science Class XII, Chapter 10.

What happens in the half second between typing a web address and seeing the page?

Your phone has no idea where the website lives. It asks a name server for the number behind the name, sends small chunks of data through a router, past an ISP, across regional and national networks, and the answer comes back the same way. Every step is a device or an address that this chapter names: a node, a modem, a switch, a router, a MAC address, an IP address, a DNS server. Learn what each one does and how it differs from its look-alike, because the exam asks exactly those differences.

What is a network?

A computer network is an interconnection of two or more computers or computing devices, so that they can share data and resources such as a printer or storage. The devices on it are called hosts or nodes: servers, desktops, laptops, mobile phones and also networking devices like switches and routers. A node is any device on the network that can receive, create, store or send data along network routes, for example a modem, hub, bridge, switch, router, printer, computer or server.

Data in a network is broken into smaller chunks called packets, which are carried over the network. Devices may be joined by wired media (cables) or wireless media (air). A hotspot made from a smartphone, with a few devices connected, is a small personal network.

How networking grew

Types of networks

Networks are classified by the area they cover and their data transfer rate.

A LAN is comparatively secure, since only authentic users can reach the shared resources, and it gives fast short range transfer. Many LANs joined together can form a MAN. A LAN joined to other LANs forms a WAN, and the Internet is the largest WAN, connecting billions of computers, smartphones and millions of LANs.

Worked example: why megabits matter. Speeds are given in Mbps, which is megabits per second, not megabytes. 1 byte is 8 bits. How long does a 50 MB file take on a 100 Mbps LAN (ignoring overheads)?

The wrong step is dividing 50 by 100 and getting half a second.

Network devices

Hub versus switch, the favourite comparison. Suppose computers A, B, C and D are connected, and A sends a packet meant for B.

Pause and predict: Four computers are connected to a hub. A sends a packet to B. How many computers other than A receive the packet at their port, and how many would receive it if a switch were used?

Check your answer: Three (B, C, D) with a hub, because it sends the packet out on all other lines. One (B) with a switch, which looks up the destination address and forwards only there.

A home Wi-Fi box is often a modem, router and switch (and wireless access point) in one: it connects to the incoming broadband line from the ISP and turns the signal into digital data your devices can use. A router is generally configured to act as the gateway, though a gateway can be software, hardware or a combination. The router moves packets between networks; the gateway is the doorway a network uses to reach others.

Beyond the textbook. For the exam: a hub broadcasts and a switch forwards to a selected device. In reality: hubs have largely disappeared from homes and offices because switches do the same job without collisions. Today's "star" networks are almost always built around a switch.

Network topologies

The arrangement of computers and other devices in a network is its topology. Five are common.

The central node of a star can be a broadcasting device (a hub, which sends to all) or a unicast device (a switch, which identifies the destination and sends only to it).

Worked example: how many cables? A computer lab has 5 computers. A fully connected mesh needs $\dfrac{n(n-1)}{2}$ cables for $n$ nodes.

Doubling to 10 computers makes the mesh need $\dfrac{10 \times 9}{2} = 45$ cables, while a star needs only 10. That growth is why mesh is costly and star is popular.

Pause and predict: In a simple bus network and in a simple ring network, one ordinary computer stops working. Which network is more likely to be disrupted?

Check your answer: The ring. Data in a ring moves one way and passes through the nodes, so a dead node or a broken link can stop the circulation. In a bus, the failed computer simply stops talking, while the shared backbone still connects the rest (a break in the backbone itself would cut the network).

Identifying nodes: MAC and IP addresses

Each node needs a unique identity so that devices can tell who sent data and who should get it.

The IPv4 address is four numbers separated by periods, each the decimal value of an 8 bit binary number, so each is between 0 and 255. A sample is 192.168.0.178. Since 32 bits give $2^{32} = 4{,}294{,}967{,}296$ addresses, just under 4.3 billion, the supply ran short as more devices came online, and IPv6 was proposed. An IPv6 address is eight groups of hexadecimal numbers separated by colons, for example 2001:CDBA:0000:0000:0000:0000:3257:9652. Eight groups of four hexadecimal digits are 32 digits, and at 4 bits each that is 128 bits.

Worked example: is it a valid IPv4 address?

Beyond the textbook. For the exam: a MAC address is permanent and cannot be changed. In reality: the number burned into a network card is permanent, but software can present a different one, and many modern phones deliberately use changing "random" addresses on Wi-Fi to make tracking harder. For the exam, the answer is still that the MAC address is permanent. The MAC is the identity of the card, and the IP address is the identity of the node on a particular network.

Beyond the textbook. For the exam: IPv6 is written as eight groups of hexadecimal digits. In reality: long IPv6 addresses are usually shortened by dropping leading zeros and replacing one run of all zero groups by a double colon, so the example above is written 2001:CDBA::3257:9652. The shortening is for people, and the address is still 128 bits.

The Internet, the Web and IoT

The Internet is the global network of computing devices: desktops, laptops, servers, tablets, phones, printers, scanners, routers, switches, gateways, and also smart TVs, ACs, lights, drones, vehicles, door locks and security cameras (the Internet of Things). A computer connects through a modem, wired or wireless, to a local ISP (Internet service provider). ISPs connect to form regional networks, regional networks join into national networks, and national networks together form the Internet backbone.

The World Wide Web (WWW) is a vast collection of interlinked web pages and web resources, accessed through the Internet. It was invented in 1990 by Sir Tim Berners-Lee, who defined three technologies.

Worked example: reading a URL. Take http://www.ncert.nic.in/textbook/textbook.htm.

A URL is not just the domain name. It carries the extra information that completes the web address.

Domain Name System (DNS)

Every server has an IP address, but numbers are hard to remember, so each server is also given a domain name (or hostname) such as wikipedia.org. The DNS works like a phone book: it matches names to numbers. Converting a domain name into its IP address is called domain name resolution, and it is done by a DNS server.

Worked example: following a request.

DNS servers are arranged in a hierarchy. At the top are 13 root servers, with other DNS servers below them. A DNS server either holds the IP address for a domain or holds the address of another DNS server where the entry can be found.

Exam traps

60-second revision

Where this chapter connects

• Previous: Structured Query Language (SQL) covers how data is stored and queried; this chapter covers how it travels.
• Next: Data Communication goes deeper into bandwidth, switching and transmission media.
• Related: Internet and Web continues the story of DNS and the Web, and Security Aspects looks at protecting networks and the data that crosses them.