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.
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. 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 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.
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 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, Google with g.co, and GoDaddy with x.co.
The main advantage of a short link is that it is, in fact, short, looks neat and clean and can be easily communicated and entered without error. To a very limited extent it may obscure the destination of the URL, though easily discoverable; this may be advantageous, disadvantageous, or irrelevant. A short link which expires, or can be terminated, has some security advantages.
A short URL obscures the target address and can be used to redirect to an unexpected site. Examples of this are rickrolling, and redirecting to shock sites, or to affiliate websites. The short URL can allow blacklisted URLs to be accessed, bypassing site blocks; this facilitates redirection of a user to blacklisted scam pages or pages containing malware or XSS attacks. TinyURL tries to disable spam-related links from redirecting. ZoneAlarm, however, has warned its users: "TinyURL may be unsafe. This website has been known to distribute spyware." TinyURL countered this problem by offering an option to view a link's destination before using a shortened URL. This ability is installed on the browser via the TinyURL website and requires the use of cookies. A destination preview may also be obtained by prefixing the word "preview" to the TinyURL URL. Other URL shortening services provide a similar destination display. Security professionals suggest that users check a short URL's destination before accessing it, following an instance where the shortening service cli.gs was compromised, exposing millions of users to security uncertainties. There are several web applications that can display the destination URL of a shortened URL.
Some URL shortening services filter their links through bad-site screening services such as Google Safe Browsing. Many sites that accept user-submitted content block links, however, to certain domains in order to cut down on spam, and for this reason, known URL redirection services are often themselves added to spam blacklists.
Another privacy problem is that many services' shortened URL format is small enough that it is vulnerable to brute-force search. Many people use URL shorteners when they share links to private content, and in fact many web services like Google Maps have offered automatic generation of shortened links for driving directions that reveal personal information like home addresses and sensitive destinations like "clinics for specific diseases (including cancer and mental diseases), addiction treatment centers, abortion providers, correctional and juvenile detention facilities, payday and car-title lenders, gentlemen’s clubs, etc."