2.1 · URL & DNS

Goal: dissect a URL and describe how DNS turns a name into an IP address.

Anatomy of a URL

https:// www.example.com :443 /path/to/page ?id=123 #section
└──┬──┘ └──────┬───────┘ └┬─┘ └────┬─────┘ └──┬──┘ └──┬───┘
 scheme    hostname     port      path     query  fragment

Part	Purpose	Example
Scheme	Protocol	http, https, ftp
Hostname	The server's name	www.example.com
Port	Numerical address of the service on the server	:80, :443 (default for HTTP/HTTPS)
Path	Location on the server	/index.html
Query	Parameters	?id=42&sort=desc
Fragment	Position within the page	#chapter-3

What DNS does

DNS (Domain Name System) is the Internet's phone book. It translates human-friendly names like www.example.com into IP addresses like 93.184.216.34.

Why we need it: people remember names; routers route to IP addresses.

A DNS lookup, step by step

1. Browser asks: "what's the IP for www.example.com?"
2. OS asks its configured DNS resolver (often the router or ISP).
3. The resolver may have it cached → returns it instantly.
4. If not, it queries the root nameservers, then .com nameservers,
   then example.com's authoritative nameservers — recursively.
5. The IP comes back and is cached for the TTL (time to live).
6. Browser opens a TCP/HTTP connection to that IP.

Common DNS record types

Record	Meaning
A	Hostname → IPv4 address
AAAA	Hostname → IPv6 address
CNAME	Alias from one name to another
MX	Mail exchange (where email goes)
NS	Authoritative nameserver for the domain
TXT	Free-form text (used for SPF, verification, etc.)

The HKEAA doesn't require record details — just the purpose of DNS.

Exam-style question

Q (4 marks): Describe what DNS is and outline the steps that occur when a user enters www.hkeaa.edu.hk in a browser.

Sample answer:

DNS (Domain Name System) is a distributed directory service that translates human-readable domain names into IP addresses.

1. The user types www.hkeaa.edu.hk.
2. The browser asks the OS's DNS resolver to translate the name.
3. The resolver checks its cache; on a miss it queries authoritative nameservers (root → .hk → edu.hk → hkeaa.edu.hk) recursively.
4. The IP address is returned and cached.
5. The browser opens a TCP connection to that IP on port 443 (HTTPS) and sends an HTTP request.

Key takeaways

• URL = scheme + host + port + path + query + fragment.
• DNS turns names into IPs.

➡️ Next: 2.2 HTTP vs HTTPS