The popular is.gd URL shortening service was reportedly unavailable for a few hours this morning, effectively breaking thousands of shortlinks posted to Twitter and other social networking sites. During the outage, the site's public-facing load balancer responded to PING requests, but was refusing HTTP connections to port 80.

is.gd is one of the most popular URL shortening services in current use – it has shortened 334 million URLs to date, which have been accessed more than 11 billion times. The service is wholly owned by UK hosting company Memset, which hosts the site on their own servers. Since December, Memset has also provided a shorter v.gd service, but this has only attracted 61 thousand URLs so far.

Memset told Netcraft that today's fault was caused by the failure of some virtual machines in the frontend cloud, which is responsible for accepting HTTP requests from the load balancer. These have been restored and the site is now back up and running with improved monitoring processes.

is.gd is primarily maintained by its creator, Richard West, a freelance developer and technologist. Memset proudly describe it as an "ethical" URL shortener; in particular, they have pledged to support is.gd as a free service indefinitely, will never place third-party adverts on the site and claim to be one of the most proactive URL shorteners in preventing spam and misuse.

Other sites hosted by Memset, including its own main presence at memset.com, were unaffected during the is.gd outage.