own custom keys. For example, can be shortened to . Not all URI schemes are capable of being shortened as of 2011, although URI schemes such as , , , , , , , , , , , , , , , and are being addressed by such services as URL shorteners. Typically, and 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 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. Expiry and time-limited services Many providers claim their shortened URLs won’t expire for as long as the service is provided. However, they may decide to discontinue the service at any time. A permanent URL is not necessarily a good thing. There are security implications, and obsolete short URLs remain in existence and may be circulated long after they cease to point to a relevant or even extant destination. Sometimes a short URL is useful simply to give someone over a telephone conversation for a one-off access or file download, and no longer needed within a couple of minutes. Some providers offer expiration on shortened URLs. This may include URLs that expire after a certain amount of time, on a certain date or after a certain number of usages. A Microsoft Security Brief recommends the creation of short-lived URLs, but for reasons explicitly of security rather than convenience. History An early reference is US Patent 6957224, which describes ...a