Cambridge O Level Computer Science · Syllabus 2210 · The Internet and its Uses
Web Server
What is Web Server?
A web server is a computer running server software that stores the web pages and other resources of one or more websites and returns them in response to HTTP or HTTPS requests from browsers.
This definition is part of the The Internet and its Uses chapter in Cambridge O Level Computer Science.
Web Server in context
The internet is the worldwide infrastructure of interconnected networks. The world wide web is the collection of websites and web pages that is accessed using that infrastructure. When you type a URL, the browser separates out the domain name, asks the domain name server (DNS) to turn it into an IP address, sends an HTTP or HTTPS request to the web server at that address, receives HTML and other resources back, and renders the page on your screen. Along the way the browser may store cookies. The same infrastructure carries digital currency transactions, which a blockchain records as time-stamped, hash-linked blocks so that a change to an old record is detectable. And because everything above is reachable from anywhere, Topic 5 finishes with the cyber security threats that exploit it and the layered solutions that reduce them.
The internet is the infrastructure: the worldwide network of interconnected computer networks, made up of the cables, fibre, routers, satellite and wireless links that carry data, together with the agreed protocols that make them work as one system. The world wide web is the collection of websites and web pages that are stored on web servers and accessed using the internet. The web is therefore one service that runs on the internet, not the internet itself.
HTTP (hypertext transfer protocol) is the set of rules that governs how a web browser and a web server exchange web pages and other resources: the browser sends a request for a named resource, and the server returns a response containing a status and, where possible, the resource itself. HTTPS (hypertext transfer protocol secure) is the same request-and-response protocol with security added: the data is encrypted before transmission, and the server proves its identity with a digital certificate. HTTPS therefore protects data in transit; it makes no claim at all about whether the website's content, owner or business is honest.
A web browser is the application software used to access and display web pages. Its main purpose is to render HTML and display web pages — that is, to interpret the HTML sent back by a web server and turn it into the formatted text, images and multimedia the user sees. Around that central job it also provides an address bar, navigation tools, multiple tabs, bookmarks or favourites, a record of user history, and the storage of cookies.
When a user enters a URL, the browser separates out the domain name and asks the domain name server (DNS) to translate it into the web server's IP address. Once it has that address the browser sends an HTTP or HTTPS request to the web server, which processes the request and returns a response containing the HTML and the other resources the page needs. The browser then renders that HTML and displays the finished page. DNS supplies an address; the web server supplies the page; the browser draws it.
Common mistakes with Web Server
- M2 — "A URL is an IP address." Why it is wrong A URL is a text-based address containing a protocol, a domain name and often a path and file name. An IP address is a numerical address identifying a device. They are connected only because DNS converts the domain name inside the URL into an IP address. Say instead "The URL contains a domain name, which the domain name server resolves to the web server's IP address."
- M3 — "Every URL must end in a file name." Why it is wrong The syllabus says a URL can contain the protocol, the domain name and the web page or file name. Many valid addresses stop at the domain name, and the server then returns its default page. Say instead "Where no file name is given, the web server supplies its default page for that address."
- M4 — "The DNS sends the web page back to the browser." Why it is wrong The domain name server returns an IP address and nothing else. The web server stores and returns the page. Say instead "The DNS returns the IP address; the browser then sends its request to the web server, which returns the HTML."
- M6 — "A browser and a search engine are the same thing." Why it is wrong The browser is application software on your device whose main purpose is to render HTML and display web pages. A search engine is a service on someone else's web servers that searches an index of pages for your keywords and returns links. The browser displays the search engine, just as it displays any other website. Say instead "The browser rendered the search engine's results page; the search engine did the searching."
- M7 — "The web server displays the page on the screen." Why it is wrong The web server sends the HTML and other resources. Turning that into what you see — rendering — happens in the browser on the user's device. Say instead "The web server returns the HTML; the browser renders it and displays the page."
Examiner tips on Web Server
- The three sentences most worth memorising word for word, because each one closes off a whole family of errors: "The world wide web is the collection of websites and web pages accessed using the internet." "The domain name server returns the IP address; the web server returns the page; the browser renders it." "Each measure reduces particular risks and leaves others open, so several complementary layers are used together."
Questions students ask about Web Server
How does a browser turn a typed URL into a displayed web page?
The browser separates the domain name out of the URL and asks the domain name server (DNS) to translate it into the web server's IP address. Once it has that address, the browser sends an HTTP or HTTPS request to the web server, which returns the HTML and other resources the page needs. The browser then renders that HTML and displays the finished page. DNS supplies an address, the web server supplies the page, and the browser draws it.