Networking, the Internet and the Web

What you need to know

• A computer network is two or more computers linked together so that they can communicate and share resources. The benefits are sharing of files and data, sharing of hardware such as one printer for a whole office, faster communication, central storage and backup, and lower cost. The drawbacks are the expense of setting it up, the speed with which a virus travels across it, the security risk of unauthorised access, and the need for skilled staff.
• By geographical coverage: a PAN covers a few metres around one person, as Bluetooth does; a LAN covers one building or campus, such as a school computer laboratory, and is privately owned and fast; a MAN covers a city, linking several branches across Lagos for instance; a WAN spans countries and continents, and the internet is the largest WAN in existence.
• In a bus topology all devices connect to a single backbone cable with a terminator at each end. It uses the least cable and is cheap to set up, but the whole network fails if the backbone is cut and performance drops sharply as more devices are added.
• In a star topology every device has its own cable to a central hub or switch. It is easy to install and to troubleshoot, and the failure of one cable or node affects only that node, but the whole network goes down if the central device fails. It uses more cable than a bus and it is the arrangement used in nearly every modern office and school laboratory.
• In a ring topology each device connects to exactly two others so that the whole forms a closed loop, and data travels in one direction, often controlled by a token. There are no collisions, but adding or removing a device disrupts the ring and a single break can bring the network down unless it is a double ring.
• In a mesh topology every device is connected to every other device. It is the most reliable, because there are many alternative routes, and the most expensive in cable and ports, which is why it is used for backbone and WAN links rather than for an office. Tree and hybrid topologies combine these basic forms.
• Transmission media are guided or unguided. Guided media are twisted pair cable, which is cheap and ends in an RJ45 connector; coaxial cable; and fibre optic cable, which carries light, offers the highest speed and longest range, is immune to electrical interference, and costs the most. Unguided media are radio waves, microwave, satellite, infrared, Bluetooth and Wi-Fi.
• Know each network device by its one distinguishing job. A network interface card gives a computer its physical connection and its MAC address. A hub receives a signal and broadcasts it to every port, which wastes bandwidth. A switch is intelligent and sends the data only to the port of the intended recipient. A router connects two different networks and chooses the path packets take. A modem converts digital signals to analogue for transmission and back again. A repeater boosts a weakened signal over distance, a bridge joins two segments of the same network, a gateway links networks using different protocols, and a firewall filters traffic to block unauthorised access.
• A network is organised either as client-server, where one powerful machine provides files, printing and authentication to the rest, or as peer-to-peer, where every machine is equal and shares directly with the others. Client-server gives central control and easier backup; peer-to-peer is cheaper for a handful of machines.
• The internet is a global network of interconnected networks communicating through the TCP/IP protocol suite. It grew out of ARPANET, which began in 1969. An intranet is a private network using the same technology but restricted to one organisation, and an extranet is an intranet extended to selected outsiders such as suppliers or partners.
• The World Wide Web is not the internet. The web is one service running on the internet: a collection of linked hypertext documents, identified by URLs and viewed through a browser. It was invented by Tim Berners-Lee at CERN in 1989. Email, file transfer and voice calls are other internet services that are not part of the web.
• Protocols to name and match: TCP/IP governs how data is broken into packets, addressed and reassembled; HTTP transfers web pages and HTTPS does the same with encryption, shown by the padlock in the address bar; FTP transfers files; SMTP sends email while POP3 and IMAP retrieve it; DNS translates a domain name such as waec.org.ng into the numeric IP address of the server.
• A URL has parts that are separately examinable. In https://www.waec.org.ng/results/index.html, https is the protocol, www.waec.org.ng is the domain name with org as the type of organisation and ng as the country code, /results is the path and index.html is the file. An IPv4 address is four numbers from 0 to 255 separated by dots, such as 192.168.1.1, while IPv6 is longer and was introduced because IPv4 addresses ran out.
• A browser is the program that fetches and displays web pages, such as Chrome, Firefox, Edge, Safari and Opera. A search engine is a website that indexes other websites and returns a list of matches, such as Google, Bing and DuckDuckGo. You use a browser to reach a search engine, and confusing the two is the single most common error in this chapter. Email addresses take the form username@domain, with Cc copying a visible recipient and Bcc copying a hidden one.