Uniform Resource Locator (URL)

Computer Networks and Internet Basics (9hrs) · Computer Fundamentals & Emerging Technologies

1. Definition

A URL (Uniform Resource Locator) is the address of a resource on the Internet — it tells the browser which protocol to use, which server to contact and which resource to fetch. Every hyperlink, bookmark, image and API call is identified by a URL. Written in a standard form defined by the IETF (RFC 3986 family), a URL is a subset of a URI (Uniform Resource Identifier): a URI identifies a resource, while a URL also says how to get it.

      scheme      authority (host + port)        path              query          fragment
        │              │                          │                  │              │
   https://www.pyqdeck.com:443/papers/cse-sem1?year=2024&subject=100108#notes

2. Components in Detail

Component	Meaning	Example from the URL above
Scheme (protocol)	How to access the resource	`https` (also http, ftp, file, mailto, tel, ssh, ws, data)
Userinfo (optional)	Credentials, rarely used and insecure	`user:pass@`
Host / domain	Server name (resolved by DNS) — may be a domain or an IP	`www.pyqdeck.com`
Port (optional)	TCP port; defaults are implied (80 for HTTP, 443 for HTTPS)	`:443`
Path	Hierarchical location of the resource on the server	`/papers/cse-sem1`
Query string	Key=value parameters sent to the server, separated by `&`	`?year=2024&subject=100108`
Fragment (anchor)	Points to a place inside the page; handled by the browser, not sent to the server	`#notes`

3. Rules, Encoding and Limits

Percent-encoding — characters not allowed in a URL (space, #, &, Devanagari, etc.) are written as % plus two hex digits: a space becomes %20, and a blank search for "engineering maths" becomes ?q=engineering%20maths.
Reserved characters (/ : ? # [ ] @ ! $ & ' ( ) * + , ; =) have special meanings and must be encoded when they are part of data.
Case sensitivity — the host/domain part is case-insensitive (WWW.PYQDECK.COM works), but the path and query are case-sensitive on most servers.
Internationalised domain names (IDN) — non-ASCII domain names are converted to Punycode: भारत.in becomes xn--h2brj9c.in.
Length — the practical limit is a few thousand characters (browsers ~2000+; specific limits apply per component).
Relative vs absolute — https://pyqdeck.com/papers/cse-sem1 is absolute; /papers/cse-sem1 or ../sem2 is relative to the current page.

4. Kinds of URLs

Type	Form	Note
Static URL	`/papers/cse-sem1`	Same content every time; cacheable
Dynamic URL	`/search?q=maths&year=2024`	Content generated from parameters
Pretty / SEO-friendly	`/cse-sem1/engineering-mathematics-1`	Human-readable, better for search engines
Deep link	App URL scheme or `https` deep path	Opens a specific screen in an app
Short URL	`bit.ly/xyz`	Redirects (301/302) to the long URL
URN	`urn:isbn:9783161484100`	Names a resource without saying how to fetch it
Other schemes	`mailto:help@pyqdeck.com`, `tel:+91...`, `file:///D:/notes.pdf`	Non-web resources

5. How the Browser Processes a URL

 1. Parse the URL into scheme, host, port, path, query, fragment
 2. Check the HSTS list / upgrade http → https if required
 3. Resolve the host name to an IP address through DNS (and cache it)
 4. Open a TCP connection to the IP on the port (443) and complete TLS
 5. Send the HTTP request:  GET /papers/cse-sem1?year=2024  Host: www.pyqdeck.com
 6. Receive and render the response; the FRAGMENT (#notes) is applied
    locally by scrolling to the matching element in the page
 7. Cache resources according to the response's cache headers

6. URL vs URI vs URN vs Other Identifiers

Term	Meaning	Example
URI	Any identifier of a resource (the superset)	`https://pyqdeck.com/x`, `urn:isbn:123`
URL	A URI that also tells how to locate the resource	`https://pyqdeck.com/x`
URN	A URI that names the resource permanently, without location	`urn:isbn:9783161484100`
Domain name	Just the server name, no protocol/path	`pyqdeck.com`
IP address	The numeric address of the machine	`104.21.32.7`

7. Security and Practical Notes

Check the scheme — https (with a padlock) means encrypted and certificate-verified, http means plain text. Be careful with typosquatting (amaz0n.com) and homograph attacks (Cyrillic а instead of Latin a in a domain), which are why browsers display Punycode for suspicious IDNs. Query parameters can leak sensitive data into logs and browser history, and open redirects in URLs are a common phishing vector. Long, complex URLs with encoded payloads are a classic sign of malicious links.

8. Exam Points

Define URL; draw and explain its parts with a labelled example — 5-7 marks, almost certain.
Explain the scheme, host, port, path, query and fragment with one example each — 5 marks, very frequent.
Differentiate a URL, a URI and a URN — 3-5 marks.
What is percent-encoding? Convert a given string with a space/& into encoded form — 3 marks.
What is a FQDN/IDN/Punycode? — 3 marks.
Trace what happens from typing a URL to the page being displayed — 5 marks.
Differentiate static and dynamic URLs; what are pretty URLs? — 3 marks.

Uniform Resource Locator (URL)