URL shortening is a technique on the World Wide Web in which a Uniform Resource Locator (URL) may be made substantially shorter and still direct to the required page. This is achieved by using a redirect which links to the web page that has a long URL. For example, the URL "https://en.wikipedia.org/wiki/URL_shortening" can be shortened to "w.wiki/U". Often the redirect domain name is shorter than the original one. A friendly URL may be desired for messaging technologies that limit the number of characters in a message (for example SMS), for reducing the amount of typing required if the reader is copying a URL from a print source, for making it easier for a person to remember, or for the intention of a permalink. In November 2009, the shortened links of the URL shortening service Bitly were accessed 2.1 billion times. Other uses of URL shortening are to "beautify" a link, track clicks, or disguise the underlying address. This is because the URL shortener can redirect to just about any web domain, even malicious ones. So, although disguising of the underlying address may be desired for legitimate business or personal reasons, it is open to abuse. Some URL shortening service providers have found themselves on spam blocklists, because of the use of their redirect services by sites trying to bypass those very same blocklists. Some websites prevent short, redirected URLs from being posted.
Purposes There are several reasons to use URL shortening. Often regular unshortened links may be aesthetically unpleasing. Many web developers pass descriptive attributes in the URL to represent data hierarchies, command structures, transaction paths or session information. This can result in URLs that are hundreds of characters long and that contain complex character patterns. Such URLs are difficult to memorize, type out or distribute. As a result, long URLs must be copied and pasted for reliability. Thus, short URLs may be more convenient for websites or hard copy publications (e.g. a printed magazine or a book). The latter often requiring that very long strings be broken into multiple lines (as is the case with some e-mail software or internet forums) or truncated. On X and some instant messaging services, there is a limit to the number of characters a message can carry. X now shortens links automatically using its own URL shortening service, t.co, so there is no need to use a separate URL shortening service just to shorten URLs in a tweet. On other such services, using a URL shortener can allow linking to web pages which would otherwise violate this constraint. 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. URL shortening sites provide detailed information on the clicks a link receives, which can be simpler than setting up an equally powerful server-side analytics engine, and unlike the latter, does not require any access to the server. URLs encoded in two dimensional barcodes such as QR code are often shortened by a URL shortener in order to reduce the printed area of the code, or allow printing at lower density in order to improve scanning reliability.
Registering a short URL Some websites create short links to make sharing links via instant messaging easier, and to make it cheaper to send them via SMS. This can be done online, at the web pages of a URL shortening service; to do it in batch via bulk upload with tools like CSV importer or on demand may require the use of an API. A few well-known websites have set up their own URL shortening services for their own use – for example, Twitter with t.co, Telegram with t.me, Google with g.co, and GoDaddy with x.co.
Techniques
In URL shortening, every long URL is associated with a unique key, which is the part after its top-level domain name. For example, https://tinyurl.com/m3q2xt has a key of m3q2xt, these keys are case-sensitive most of the time and using the wrong case may lead to a different destination URL. Not all redirection is treated equally; the redirection instruction sent to a browser can contain in its header HTTP response status codes such as 301 (moved permanently), 302 (found), 307 (temporary redirect) or 308 (permanent redirect). There are several techniques to implement 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 custom keys. For example, https://example.com/product?ref=01652&type=shirt can be shortened to https://tinyurl.com/exampleshirt. Not all URI schemes are capable of being shortened as of 2011, although URI schemes such as http, https, ftp, ftps, mailto, mms, rtmp, rtmpt, ed2k, pop, imap, nntp, news, ldap, gopher, dict and dns are being addressed by such services as URL shorteners. Typically, data: and javascript: URLs are not supported for security reasons, to combat attacks like cross-site scripting and session hijacking. Some URL shortening services support the forwarding of mailto URLs, as an alternative to address munging, to avoid unwanted harvest by web crawlers or bots. This may sometimes be done using short, CAPTCHA-protected URLs, but this is not common. Makers of URL shorteners usually register domain names with less popular or esoteric Top-level domains in order to achieve a short URL and a catchy name, often using domain hacks. This results in registration of different URL shorteners with a myriad of different countries, leaving no relation between the country where the domain has been registered and the URL shortener itself or the shortened links. Top-level domains of countries such as Libya (.ly), Samoa (.ws), Mongolia (.mn), Malaysia (.my) and Liechtenstein (.li) have been used as well as many others. In some cases, the political or cultural aspects of the country in charge of the top-level domain may become an issue for users and owners, but this is not usually the case. Services may record inbound statistics, which may be viewed publicly by others.
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