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URL shortening

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URL shortening is a technique on the World Wide Web in which a Uniform Resource Locator (URL) may be made substantially shorter in length and still direct to the required page. This is achieved by using an HTTP Redirect on a domain name that is short, which links to the web page that has a long URL. For example, the URL http://en.wikipedia.org/wiki/URL_shortening can be shortened to http://bit.ly/urlwiki. This is especially convenient for messaging technologies such as Twitter and Identi.ca, which severely limit the number of characters that may be used in a message. Short URLs allow otherwise long web addresses to be referred to in a tweet. In November 2009, the shortened links on one URL shortening service were accessed 2.1 billion times.

Normally, a URL shortening service will use the top-level domain of a country that allows foreign sites to use its extension, such as .ly or .to (Libya and Tonga), to redirect worldwide using a short alphanumeric sequence after the provider's site address in order to point to the long URL. This use of foreign domains has been criticized as a weakness in the business model of some service providers.

Another use of URL shortening is to disguise the underlying address. Although this may be desired for legitimate business or personal reasons, it is open to abuse and for this reason, some URL shortening service providers have found themselves on spam blacklists, because of the use of their redirect services by sites trying to bypass those very same blacklists. Some websites prevent short, redirected URLs from being posted.

Contents 1 Purposes 2 Registering a short URL 3 Techniques 4 History 5 Criticism and problems 5.1 Linkrot 5.2 Closure by Internet service provider 5.3 International law 5.4 Blocking 5.5 Privacy and security 5.6 Additional layer of complexity 6 See also 7 References 8 External links

Purposes

There are several reasons to use URL shortening. The free hosting space provided by access Internet Service Providers for its customers may generate an aesthetically unpleasing address. Some web developers on mainstream sites tend to pass descriptive attributes in the URL to represent data hierarchies, command structures, transaction paths or session information and this often results in a URL that contains a large number of characters, is awkward to reproduce and impossible to remember. Copying a URL that is hundreds of characters long can only really be successfully done by copy-and-paste. Trying to type one by hand will be time-consuming and result in errors. Thus a short URL is more useful to write in an e-mail message or an internet forum post.

On Twitter and some instant-messaging services, there is a limit on the total number of characters that can be used in a message. Using a URL shortener can make it easier to include a URL within a short message. Some shortening services, such as tinyurl.com and bit.ly, can generate URLs that are human-readable, although the resulting strings are longer than those generated by a length-optimized service. A QR Code that stores a URL can be more readable by using a URL shortener to minimize the length of the URL it encodes.

Registering a short URL

An increasing number of websites are registering their own short URLs to make sharing via Twitter and SMS easier. This can normally be done online, at the web pages of a URL shortening service. Short URLs often circumvent the intended use of top-level domains for indicating the country of origin; domain registration in many countries requires proof of physical presence within that country, although a redirected URL has no such guarantee.

Techniques

See also: URL redirection

In URL shortening, every long URL is associated with a unique key, which is the part after http://top-level domain name/, for example http://tinyurl.com/m3q2xt has a key of m3q2xt. Not all redirection is treated equally; the redirection instruction sent to a browser can contain in its header the HTTP status 301 (permanent redirect) or 302 (temporary redirect). In the field of search engine optimization it is believed that a permanent redirect will receive a higher ranking.

There are several techniques to implement a URL shortening. Keys can be generated in base 36, assuming 26 letters and 10 numbers. In this case, each character in the sequence will be 0, 1, 2, ..., 9, a, b, c, ..., y, z. Alternatively, if uppercase and lowercase letters are differentiated, then each character can represent a single digit within a number of base 62 (26 + 26 + 10). In order to form the key, a hash function can be made, or a random number generated so that key sequence is not predictable. Or users may propose their own keys. For example, http://en.wikipedia.org/w/index.php?title=TinyURL&diff=283621022&oldid=283308287 can be shortened to http://bit.ly/tinyurlwiki.

Not all protocols are capable of being shortened, as of 2011, although protocols such as http, https, ftp, ftps, mailto, news, mms, rtm

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