Locating Resources on the World Wide Web (Cambridge (CIE) A Level Computer Science) : Revision Note
URL
What is a URL?
A Uniform Resource Locator (URL) is a unique identifier for a web page, known as the website address
It is text-based to make it easier to remember
A user enters a URL into a web browser to view a web page
An example of a URL is:
A URL can typically be split into three parts:
Protocol
Domain name
Web page/file name
Using the example about the URL would be split as follows:
Protocol | https | Communication method to transfer data between client and server |
|---|---|---|
Domain name | Name of the server where the resource is located | |
Web page/file name | /a-level/computer-science/ | Location of the file or resources on the server |
DNS
What is the Domain Name System (DNS)?
The DNS is like the Internet’s phone book
It translates domain names (like
www.google.com) into IP addresses (like142.250.180.68)Computers use IP addresses to find and connect to servers
When you type a URL into your browser:
DNS finds the matching IP address
Your device connects to that server to load the website
Without DNS, users would have to remember the IP address of every website they visit
When a domain is registered or its server IP address changes, the DNS must be updated
This update is called DNS propagation, and it may take some time to spread across the internet
What happens when you type a URL into a web browser?
The user enters a URL into the address bar of the web browser
The browser checks its cache to see if it already knows the IP address for the website
If not found, it sends the domain name to a DNS server, which stores an index of domain names and their matching IP addresses
If the DNS server finds the IP address, it sends it back to the web browser
If it does not find the IP address, it passes the request on to a higher-level DNS server
This may involve contacting:
A root server, which points to the correct Top-Level Domain (TLD) server (e.g. .com, .org),
The TLD server, which points to the authoritative DNS server for the domain
The authoritative DNS server responds with the correct IP address
The web browser then sends a request to the web server at that IP address
The web server processes the request and sends back the website’s data (such as HTML, images, CSS, and JavaScript)
Finally, the web browser renders the content and displays the website to the user
Worked Example
Complete the following paragraph that describes the sequence of events when a user requests a page from a website.
The user enters the Uniform Resource Locator (URL) into the address bar of the ......................................... .
The domain name is taken from the URL and sent to a Domain Name Service (DNS). This stores .................................................. of domain names and their matching IP addresses.
If it finds the domain name, it sends the IP address to the .................................................. .
If it does not find the domain name, it sends the request to ..................................................
Answer
Web browser [1 mark]
An index/a list/a table [1 mark]
Web browser [1 mark]
A higher-level DNS [1 mark]
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