The internet and its uses

The internet, the world wide web and the URL

The internet is the infrastructure. It is the global network of interconnected networks, made up of cables, routers, switches and servers, over which data travels from one device to another. It exists whether or not anybody is looking at a web page, and it carries many different services.

The world wide web is the collection of websites and web pages accessed using the internet. The web is one service that runs on the internet, alongside others such as email, file transfer and video calls. So the internet is the thing that connects the computers, and the world wide web is the collection of pages you visit over that connection.

A useful way to hold the two apart is a road network. The internet is the roads, and the world wide web is the shops and buildings you can reach by driving on them. You can use the roads without going to a shop, and the shops cannot exist without the roads. In the same way, you can be connected to the internet and never open a web page, but a web page cannot be delivered to you unless you are connected to the internet.

A uniform resource locator (URL) is a text-based address for a web page. It tells the browser where the page is and how to fetch it. A URL can contain three parts: the protocol, the domain name and the web page or file name.

Take https://www.example.com/revision/physics.html as an example. The protocol is https, which tells the browser that the page must be requested using hypertext transfer protocol secure. The domain name is www.example.com, which identifies the web server that holds the site, in a form a person can read and remember. The file name is /revision/physics.html, which identifies the particular page within that site.

Not every URL shows all three parts. The protocol is sometimes left out, and the browser fills it in; the file name is sometimes left out, and the server then returns a default page for the site, which is often called index.html. The domain name is the part that always has to be there, because without it the browser has no idea which server to ask.

• The internet is the infrastructure: the global network of interconnected networks that data travels over.
• The world wide web is the collection of websites and web pages accessed using the internet.
• The web is one service that runs on the internet, alongside email, file transfer and video calls.
• You can use the internet without using the web, but you cannot use the web without the internet.

HTTP and HTTPS

Hypertext transfer protocol (HTTP) is the set of rules that a web browser and a web server use to communicate. It is a request-response protocol: the browser sends a request for a resource, and the server sends back a response. The request contains the address of the page being asked for, and the response contains the page itself, written in HTML, along with a status code saying whether the request succeeded.

The purpose of HTTP is to make the web work between any browser and any server. Because the rules are standard, a browser written by one company can request pages from a server run by another, and neither has to know anything about the other. Without an agreed protocol, every browser would only be able to talk to servers built by the same company.

Hypertext transfer protocol secure (HTTPS) is HTTP with encryption. The data sent between the browser and the server is scrambled, so anyone who intercepts it cannot read it, and the site proves its identity with a digital certificate. The purpose is to protect information that must not be seen by anyone else, such as passwords, card details and personal messages, and to let the user check that they are connected to the genuine site rather than an imitation of it.

The two protocols work in the same way from the user's point of view: the browser still sends a request and the server still sends a response. The difference is that with HTTPS the content of that request and response is encrypted in transit, and the browser usually shows a padlock in the address bar to tell the user that the connection is secure. Browsers now warn users about pages that use plain HTTP, especially on any page that asks for a password or payment.

The web browser: purpose and functions

The main purpose of a web browser is to render HTML and display web pages. Rendering means taking the HTML file that comes back from the web server and turning it into the page you actually see: headings, paragraphs, images and links laid out on the screen. A browser is application software, and it is the program that turns the code of a page into something a person can read and click.

The address bar is where the URL is typed or shown, and it is also where the browser reports whether the connection is secure. Navigation tools are the back, forward, refresh and home buttons, which let the user move between pages they have already visited and reload a page that has not displayed properly. Multiple tabs let the user have several pages open at once in the same window and switch between them, instead of opening a separate window for each one.

Bookmarks and favourites let the user save the address of a page so that they can return to it later without typing the URL again, usually organised into folders. History is the record of the pages the user has visited, with the time and date, which makes it possible to find a page again when the address has been forgotten. Cookies are small text files that the browser stores on behalf of a website, and the browser sends them back to that site when the user returns to it. Cookies have a block of their own below, because the syllabus examines them separately.

From URL to screen: DNS, IP addresses, the web server and HTML

When a user enters a URL, the browser has the address in a form written for people, but the network needs an address in a form it can use. Every device on a network is identified by an IP address, which is what is actually used to send data to the right place. A domain name is far easier for a person to remember and type than a string of numbers, so something has to translate between the two.

That translation is done by the domain name server (DNS). The DNS is a system of servers that hold a directory matching domain names to IP addresses. The browser asks the DNS for the IP address of the domain name in the URL, and the DNS returns it. Once the browser has the IP address, it knows which machine to send the request to.

The browser then sends an HTTP or HTTPS request to that IP address. The machine at the other end is the web server, the computer that stores the files that make up the website and serves them out on request. The web server finds the page or file named in the request and sends it back to the browser. Large websites are usually held on servers in a data centre, and a site may be hosted on servers provided as part of a larger cloud platform rather than on one machine the owner runs, but the job is the same either way: store the files and serve them when asked.

What comes back is HTML, hypertext markup language, the language web pages are written in. HTML uses tags to mark up the structure of a page, saying which text is a heading, which is a paragraph, where an image goes and which words are links. The browser renders that HTML into the page on screen, and fetches any images or other files the page refers to in the same way.

The whole sequence is worth learning in order, because exam questions ask for it step by step: the user enters the URL, the browser asks the DNS for the IP address of the domain name, the DNS returns the IP address, the browser sends a request to the web server at that IP address, the web server sends back the HTML for the page, and the browser renders the HTML and displays the page.

• The URL is entered in the browser, which needs an IP address rather than a domain name.
• The browser asks the domain name server (DNS), which returns the IP address of the domain name.
• The browser sends an HTTP or HTTPS request to that IP address.
• The web server stores the site's files, finds the page requested and sends the HTML back.
• The browser renders the HTML and displays the web page on the screen.

Cookies: session and persistent

A cookie is a small text file that a website stores on the user's device, and that the browser sends back to that same website the next time the user requests a page from it. Cookies exist because HTTP itself keeps no memory of previous requests: each request arrives at the server on its own, so without a cookie the server could not tell that the second request came from the same person as the first. A cookie is data, not a program, so it cannot run code or carry a virus on its own.

Cookies are used for a number of jobs. They save personal details, so a form such as a delivery address can be filled in automatically next time. They track user preferences, such as the language, text size or layout the user chose, so the site looks the same on the next visit. They hold the items in an online shopping cart, so the basket is still there when the user moves from one page to another or returns the next day. And they can store login details, so a user who asked to be remembered does not have to type a username and password on every visit.

A session cookie exists only while the browser is open. It is created when the user arrives at the site and deleted when the browser is closed, and it is what keeps one visit coherent: it is how the shopping site remembers what is in the basket as the user moves from page to page. A persistent cookie is stored on the device with an expiry date and survives closing the browser, so it can remember the user between visits, which is what keeps them logged in and keeps their preferences in place.

Digital currency and blockchain

A digital currency is one that only exists electronically. There are no coins and no notes: the balance is held as data in a computer system, and paying with it means transferring that data from one account to another. Nothing physical changes hands, and there is no central pile of cash anywhere.

Because the currency exists only as data, the record of who owns what has to be secure and trusted. That record is kept using a blockchain. In its basic form a blockchain is a digital ledger: a time-stamped series of records that cannot be altered. Transactions are grouped together into blocks, each block is stamped with the time it was added, and each block is linked to the block before it, so the blocks form a chain.

This is how a blockchain is used to track digital currency transactions. Every transaction is written into the ledger, and because each block is linked to the one before it, changing a record in an old block would break the chain and be visible. The ledger is shared rather than held in one place by one organisation, so no single party can quietly rewrite the history of who paid whom. The result is a record of transactions that all the users of the currency can check, and the same digital currency cannot be spent twice.

Two words to keep straight. The ledger is the record of transactions; the blockchain is the way that record is built and secured, out of time-stamped blocks that cannot be altered once they have been added.

• A digital currency exists only electronically: the balance is data, not coins or notes.
• A blockchain is a digital ledger, a time-stamped series of records that cannot be altered.
• Transactions are grouped into blocks, and each block is linked to the block before it.
• The ledger is shared, so the record of transactions is not held by one single organisation.
• Because the records cannot be altered, the same digital currency cannot be spent twice.

Cyber security: the threats

A cyber security threat is any attempt to gain unauthorised access to a system or data, to damage it or to make it unavailable. Some threats work by attacking the technology and others work by attacking the person using it, and the two need different defences.

Hacking is gaining unauthorised access to a computer system, with the aim of stealing data, altering it or disrupting the system. A brute-force attack is one way in: the attacker tries every possible combination of characters until the password is found, which is why long passwords made of different types of character are harder to break. Data interception is capturing data while it is being transmitted, for example by tapping into a network or intercepting a wireless signal, with the aim of reading or stealing it. A distributed denial of service (DDoS) attack floods a server with requests sent at the same time from a large number of compromised computers, so that the server cannot respond to genuine users: the aim is not to steal data but to make the service unavailable.

Malware is malicious software, written to damage a system or gain access to it. The named types behave differently, and exam questions expect you to know how each one spreads and what it does.

The remaining threats work on people rather than on machines. Phishing is sending a message that pretends to come from a trusted organisation and asks the user to click a link or enter personal details on a fake page, with the aim of tricking the user into giving away passwords or card details. Pharming redirects a user to a fake website without their knowledge, for example by altering the settings that turn a domain name into an IP address, so that typing the correct address still lands the user on the attacker's copy of the site. Social engineering is manipulating people into giving out confidential information or granting access by gaining their trust, such as telephoning someone and pretending to be from the IT department. None of these three needs a technical weakness in the software: they need a person who believes the story.

• Hacking: unauthorised access to a system.
• Brute-force attack: trying every possible combination of characters until the password is found.
• Data interception: capturing data while it is being transmitted.
• DDoS: flooding a server with requests from many compromised computers so genuine users cannot reach it.
• Phishing, pharming and social engineering: attacking the person rather than the software.
• Malware: virus, worm, Trojan horse, spyware, adware and ransomware.

Cyber security: keeping data safe

There is no single defence against every threat, so solutions are used in layers, each one covering a different way in. Some of them control who is allowed to do what, some protect the system from malicious software, and some rely on the user noticing that something is wrong.

Access levels give each user permission only for the files and settings their role needs, so an account that is compromised cannot reach everything on the system. Authentication checks that a user is who they claim to be, and it can be based on something the user knows, such as a username and password, on something the user is, such as a fingerprint or a face recognised by biometrics, or on two different pieces of evidence used together, which is two-step verification. Two-step verification means a stolen or guessed password on its own is not enough to log in.

Anti-malware software, which includes anti-virus and anti-spyware, scans a system for malicious software and removes it, and it is much more effective when software updates are automated, because updates close the known weaknesses that malware uses to get in. A firewall monitors and filters the data entering and leaving a network or a computer against a set of rules, blocking traffic that should not be there. A proxy server sits between the user and the internet, so requests are made through it: it can hide the user's IP address from the sites being visited and filter what is requested.

Privacy settings control what a user shares and who can see it, which limits how much personal data is exposed in the first place. Two further defences need no software at all. Checking the spelling and tone of communications catches messages that pretend to be from a trusted organisation but are full of errors or written in an odd tone, which is what a phishing message usually looks like. Checking the URL attached to a link before clicking it matters because the words displayed on the page can be made to read like a trusted address while the actual URL leads somewhere else entirely.

• Control who can do what: access levels, strong authentication and privacy settings.
• Protect the system: anti-malware, automated software updates, a firewall and a proxy server.
• Protect yourself: check the spelling and tone of messages, and check the URL behind a link before clicking it.