Explain how DNS resolves domain names to IP addresses, and describe the HTTP/HTTPS request-response model and the client-server architecture

What this dot point is asking

SEAB wants you to explain how DNS resolves domain names to IP addresses, and to describe the HTTP/HTTPS request-response model and the client-server architecture. The central idea is that the web works by name lookup followed by a request and response: DNS turns a name into an address, then a client asks a server for a resource and the server replies.

The answer

The Domain Name System

Computers route to each other by numeric IP addresses, but people use memorable domain names like example.com. The Domain Name System (DNS) translates names to IP addresses. It is:

• distributed - records are spread across many servers worldwide, not held in one place, and
• hierarchical - the namespace has levels: root servers, top-level domain (TLD) servers (.com, .sg), and authoritative servers for individual domains.

Resolving a name

When a browser needs the IP for a domain it has not cached:

1. It asks a DNS resolver (often the ISP's).
2. The resolver, if it has no cached answer, queries a root server, which points to the right TLD server, which points to the domain's authoritative name server.
3. The authoritative server returns the IP address; the resolver caches it (for a set time) and returns it to the browser.

Caching at each level means most lookups are answered quickly without the full chain.

The client-server model

The web uses a client-server architecture: the client (browser) requests resources, and the server stores and serves them. Many clients share one server. Communication follows a request-response cycle.

HTTP request-response

HTTP is the application-layer protocol of the web:

Client request:  GET /index.html  (method + path + headers)
Server response: 200 OK + the HTML content (status + body)

The client sends a request (a method such as GET to read or POST to send data, plus the path); the server returns a status code and usually the content. HTTP is typically stateless - each request stands alone.

Status code categories: 2xx success (200 OK), 3xx redirection, 4xx client error (404 Not Found), 5xx server error (500).

HTTPS

HTTPS is HTTP over an encrypted TLS connection. It adds:

• Confidentiality - eavesdroppers cannot read the data.
• Integrity - data cannot be tampered with undetected.
• Authentication - a certificate confirms the server is genuine.

This protects passwords, payments and private data, which is why it is now standard.

Examples in context

Example 1. Why a site loads faster the second time. The first visit triggers a full DNS lookup; afterwards the resolver and browser cache the IP, so subsequent visits skip the lookup chain. Caching is what makes repeat visits noticeably quicker, and explains the brief delay only on a brand-new domain.

Example 2. The padlock and online payments. A checkout page uses HTTPS so card details are encrypted in transit and the certificate proves you are talking to the real bank, not an impostor. The same HTTP request-response cycle applies, but TLS underneath guarantees no eavesdropper can read or alter the exchange.

Try this

Q1. What does DNS translate, and into what? [1 mark]

• Cue. It translates human-readable domain names into numeric IP addresses.

Q2. Describe the request-response cycle of HTTP. [2 marks]

• Cue. The client sends an HTTP request (method and path) to the server; the server returns a response with a status code and usually the requested content.

Q3. State two things HTTPS adds over HTTP. [2 marks]

• Cue. Confidentiality (encryption) and integrity, plus server authentication via a certificate.