URL - Uniform Resource Locator

HTTP on a computer monitor
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URL stands for Uniform Resource Locator. A URL is a formatted text string used by Web browsers, email clients and other software to identify a network resource on the Internet. Network resources are files that can be plain Web pages, other text documents, graphics, or programs.

URL strings consist of three parts (substrings):

  1. protocol designation
  2. host name or address
  3. file or resource location

These substrings are separated by special characters as follows:

protocol :// host / location

URL Protocol Substrings

The 'protocol' substring defines a network protocol to be used to access a resource. These strings are short names followed by the three characters '://' (a simple naming convention to denote a protocol definition). Typical URL protocols include HTTP (http://), FTP (ftp://), and email (mailto://).

URL Host Substrings

The 'host' substring identifies a destination computer or other network device. Hosts come from standard Internet databases such as DNS and can be names or IP addresses. The host names of many Web sites refer to not just a single computer but rather groups of Web servers.

URL Location Substrings

The 'location' substring contains a path to one specific network resource on the host. Resources are normally located in a host directory or folder. For example, some Web sites may have a resource like /2016/September/word-of-the-day-04.htm to organize content by dates. This example shows a resource having two subdirectories and a file name.

When the location element is an empty, shortcut as in the URL http://thebestsiteever.com, the URL conventionally points to the root directory of the host (denoted by a single forward slash - '/') and often a home page (like 'index.htm').

Absolute versus Relative URLs

Full URLs featuring all three of the abovementioned substrings are called absolute URLs. In some cases, URLs may specify only the one location element. These are called relative URLs. Relative URLs are used by Web servers and Web pages to reduce the length of URL strings.

Following the above example, Web pages on the same that link to it can code a relative URL

<a href="/2016/September/word-of-the-day-04.htm">

instead of the equivalent absolute URL

<a href="http://thebestsiteever.com/2016/September/word-of-the-day-04.htm">

taking advantage of the Web server's ability to automatically fill in the missing protocol and host information. Note that relative URLs can only be used in cases like this where the host and protocol information is established.

URL Shortening

Standard URLs on modern Web sites tend to be long strings of text. Because sharing longer-length URLs on Twitter and other social media is cumbersome, several companies built online translators that convert a full (absolute) URL into a much shorter one specifically for use on their social networks. Popular URL shorteners of this kind include t.co (used with Twitter) and lnkd.in (used with LinkedIn).

Other URL shortening services like bit.ly and goo.gl work across the Internet and not just with specific social media sites.

In addition to offering an easier way to share links with others, some URL shortening services also offer click statistics. A few also safeguard against malicious uses by checking the URL location against lists of suspicious Internet domains.