You type a URL and a page appears in a fraction of a second. Behind it, the browser has looked up an IP address, asked a web server for files and turned HTML into a page. This is one of the most common six-mark questions, so learn the steps in order.
Computers on the internet are found by IP address, not by name. When a URL is entered, the browser sends the domain name to a domain name server (DNS), which looks up and returns the matching IP address. The browser then sends an HTTP request to the web server at that IP address. The web server sends back the page as HTML files, and the browser renders the HTML to display the page.
Fig. 1: the DNS turns the domain name into an IP address, then the web server sends the HTML
Section 01
The steps in order.
Run the lab from start to finish. Each step lights up as it happens: the DNS lookup, the request to the web server, the response, and the rendering of the page.
1. The user types a URL into the address bar of the web browser (or clicks a hyperlink).
2. The browser sends the domain name part of the URL to a DNS.
3. The DNS searches its database for the domain name and finds the matching IP address.
4. The DNS sends the IP address back to the browser.
5. The browser sends an HTTP(S) request to the web server at that IP address.
6. The web server finds the page and sends its files, written in HTML, back to the browser.
7. The browser renders the HTML and displays the web page.
Examiner trap
The DNS does not send the web page. It only returns the IP address. The web server sends the HTML. Mixing these up loses most of the marks.
Section 02
When the DNS does not know.
No single DNS holds every domain name in the world. DNS servers are arranged so that they can pass a query on.
Not found locally: if a DNS does not have the domain name in its database, it passes the request to another DNS higher up.
Found: the IP address is passed back down to the browser (and the first DNS can store it to answer faster next time).
Never found: if no DNS holds the domain name, an error message is returned to the browser, saying the site cannot be found.
Worked example Exam-style answer
Describe the role of the DNS when a user enters www.example.com.
The browser sends the domain name www.example.com to the DNS.
The DNS looks up the domain name in its database to find the matching IP address; if it does not hold it, it asks another DNS.
The DNS returns the IP address to the browser, which can now contact the web server.
Answer: The DNS translates the domain name into the IP address of the web server and sends it back to the browser.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
What is the role of the domain name server (DNS)?
Quick check+10 XP
In the lab, what does the DNS send back to the browser?
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What does the web server send to the browser?
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What happens if a DNS does not have the domain name in its database?
Section 03
Who does what.
Questions often ask for the role of one part. Match each part to its single job.
Part
Role
Web browser
Takes the URL, contacts the DNS, sends the request to the web server, renders the HTML and displays the page
IP address
The unique numeric address used to locate the web server on the internet
Domain name server (DNS)
Stores domain names with their IP addresses; returns the IP address for a domain name
Web server
Stores the website's files; sends the requested web page to the browser
HTML
The language the page is written in; the browser renders it to display the page
Your turn
Put these in order: web server sends HTML; DNS returns IP address; browser renders page; browser sends domain name to DNS; browser requests page from IP address.
Browser sends domain name to DNS; DNS returns IP address; browser requests page from IP address; web server sends HTML; browser renders page.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 1 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which is the correct order of events after a URL is entered?
Exam style+20 XP
Why does the browser need the IP address of the web server?
Exam style+20 XP
Which component renders HTML to display a web page?
Exam style+20 XP
Which part of the URL is sent to the DNS?
Skill unlocked
Locating and retrieving a web page, mastered.
This skill is now recorded on your profile. Keep the chain going.