Why this matters
Two of the most-used services on the internet are the web and email. Both feel instant, but behind each click a precise sequence of lookups and protocols runs. Understanding that sequence turns "magic" into something you can reason about and troubleshoot.
The idea
The WWW (World Wide Web, or just the Web) links content all over the internet so you can browse
web pages. Pages are written in HTML, and each page has a URL that says where it is and
how to reach it. A URL like https://www.mext.go.jp/index.html breaks into a scheme (https), a
host name (www), the domain (mext.go.jp, made of organization name + organization type go +
country code jp), and a file name (index.html).
To load a page, your browser first asks DNS to turn the domain name into an IP address, then contacts that web server, which sends back the page:
- Browser queries the DNS server for the IP address.
- DNS server returns the IP address.
- Browser accesses the web server using that IP address.
- Web server sends the page, and the browser displays it.
Email lets you exchange messages with specific people. An email address has the form
username@domain: the username identifies the recipient and the domain name identifies
their mail server. Three protocols move mail: SMTP sends mail to a server, POP
downloads mail and deletes it from the server, and IMAP lets you view mail kept on the
server.
Picture it
sequenceDiagram participant B as Browser participant D as DNS server participant W as Web server B->>D: 1. What is the IP for this domain? D-->>B: 2. Here is the IP address B->>W: 3. Request page at that IP W-->>B: 4. Send page, browser displays it
Worked example
You type https://www.example.co.jp and the page appears. What order do the steps run in? First the
browser must turn the domain into a number, so it queries DNS; DNS returns the IP address;
the browser accesses that IP; finally the web server sends the page. For email, which
protocol sends and which receives? SMTP sends to the mail server, while POP or IMAP
receives: POP downloads and removes, IMAP leaves the mail on the server to view.
Your turn
Try the practice questions. Match each protocol or part to its role, and sort web tasks from email tasks.
Recap
- The WWW serves HTML pages located by a URL; DNS turns the domain into an IP first.
- An email address is
username@domain. - SMTP sends; POP downloads and deletes; IMAP views mail on the server.