URL

A Uniform Resource Locator (URL), colloquially termed a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it

• Defined in RFC 1738 in 1994 by Tim Berners-Lee, a URL is a specific type of Uniform Resource Identifier (URI) (RFC 3986)

• URL is a subset of URI

URLs are used to reference:

• Web pages (http)
• File transfer (ftp)
• email (mailto)
• Database access (JDBC)
• And many other applications

URL Syntax

Every HTTP URL conforms to the syntax of a generic URI (consisting of a hierarchical sequence of 5 components):

URI = scheme ":" ["//" authority] path ["?" query] ["#" fragment]

Where the authority component divides into 3 subcomponents:

authority = [userinfo "@"] host [":" port]

The URI comprises:

1. Scheme: A non-empty scheme component followed by a colon (:):

   • Begins with a letter
   • Followed by any combination of letters, digits, plus (+), period (.), or hyphen (-)
   • Canonical form lowercase (they are case-insensitive)
   • E.g. http, https, ftp, mailto, file, data, and irc

2. Authority: An optional component preceded by 2 slashes (//):

   • userinfo (optional) subcomponent that may consist of a user name and an optional password proceeded by a colon (:), followed by an at symbol (@)

     • username:password: is depreciated for security reasons (it uses plain text)

   • host: A subcomponent consisting of either a registered name or an IP address

     • IPv4 addresses must be in dot-decimal notation
     • IPv6 addresses must be enclosed in brackets ([])

   • port: optional subcomponent preceded by a colon (:)

3. Path: A component consisting of a sequence of path segments separated by a slash (/)

   • Always defined and may be empty (0 length)
   • In http and https URIs, the last part of a path is named pathinfo (optional)
   • For more information Wiki URL

4. Query: An optional component preceded by a question mark (?) containing a query string of non-hierarchical data:

   • Its syntax is not well-defined
   • By convention is often a sequence of attribute–value pairs separated by a delimiter.

5. Fragment: An optional component proceeded by a hash (#):

   • The fragment contains a fragment identifier providing direction to a secondary resource, such as a section heading in an article identified by the remainder of the URI.
   • When the primary resource is an HTML document, the fragment is often an id attribute of a specific element, and web browsers will scroll this element into view.

Example URL http://www.example.com:81/a/b.html?user=Alice&year=2049#heading is divided into different parts:

1. Scheme: http (Protocol)

2. Authority: Domain Name + Port

   • Domain Name: www.example.com (Hostname)
   • Port Number: 81

3. Path: /a/b.html (Path to the file or API endpoints or dynamically generated)

4. Query: ?user=Alice&year=2049 (Parameters)

5. Fragment: #heading (Anchor)

Ways to specify a URL

1. Full URL: <a href='http://stanford.edu/news/2019/'>2019 News</a>

2. Relative URL: <a href='september'>September News</a>

   • Same as: http://stanford.edu/news/2019/september

3. Absolute URL: <a href='/events'>Events</a>

   • It removes everything till the root and adds /event to it, unlike relative URL
   • Same as: http://stanford.edu/events

4. Fragment URL: <a href='#section3'>Jump to Section 3</a>

   • Scrolls to <a href='section3' /> within page
   • Same as: http://stanford.edu/events#section3

Some HTML tags that can include URLs:

• <img>
• <video>, <audio>
• <canvas>
• <link>, <style>
• <script>

TRAILING FORWARD SLASH

If you add trailing / to the URL?

Internationalized URL

An Internationalized Resource Identifier (IRI) is a form of URL that includes Unicode characters. All modern browsers support IRIs

• The domain name in the IRI is known as an Internationalized Domain Name (IDN)

• Web and Internet software automatically convert the domain name into punycode usable by the Domain Name System; for example, the Chinese URL http://例子.卷筒纸 becomes http://xn--fsqu00a.xn--3lr804guic/. The xn-- indicates that the character was not originally ASCII

  • Punycode is a representation of Unicode with the limited ASCII character subset used for Internet hostnames

• The user can also specify the URL path name in the local writing system. If not already encoded, it is converted to UTF-8, and any characters not part of the primary URL character set are escaped as hexadecimal using percent-encoding;

  • For example, the Japanese URL http://example.com/引き割り.html becomes http://example.com/%E5%BC%95%E3%81%8D%E5%89%B2%E3%82%8A.html. The target computer decodes the address and displays the page

References

• Punycode converter