Locating Resources on the World Wide Web (Cambridge International A-Level Computer Science)

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:

  • https://www.savemyexams.com/a-level/computer-science/

• 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:

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 (like 142.250.180.68)

• 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?

1. The user enters a URL into the address bar of the web browser
2. The browser checks its cache to see if it already knows the IP address for the website
3. If not found, it sends the domain name to a DNS server, which stores an index of domain names and their matching IP addresses
4. If the DNS server finds the IP address, it sends it back to the web browser
5. If it does not find the IP address, it passes the request on to a higher-level DNS server
6. 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

1. The authoritative DNS server responds with the correct IP address
2. The web browser then sends a request to the web server at that IP address
3. The web server processes the request and sends back the website’s data (such as HTML, images, CSS, and JavaScript)
4. 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.

1. The user enters the Uniform Resource Locator (URL) into the address bar of the ......................................... .
2. 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.
3. If it finds the domain name, it sends the IP address to the .................................................. .
4. If it does not find the domain name, it sends the request to ..................................................

Final answer: Answer

1. Web browser [1 mark]
2. An index/a list/a table [1 mark]
3. Web browser [1 mark]
4. A higher-level DNS [1 mark]