Via Slash Gear

Everyone on the internet
has come across at least couple error codes, the most well-known being
404, for page not found, while other common ones include 500, for
internal server error, or 403, for a "forbidden" page. However, with
latter, there's the growing issue of why a certain webpage has become
forbidden, or who made it so. In an effort to address things like
censorship or "legal obstacles," a new code has been published, to be
used when legal demands require access to a page be blocked: error 451.
The number is a knowing reference Fahrenheit 451,
the novel by Ray Bradbury that depicted a dystopian future where books
are banned for spreading dissenting ideas, and in burned as a way censor
the spread of information. The code itself was approved for use by the
Internet Engineering Steering Group (IESG), which helps maintain
internet standards.
The idea for code 451 originally came about around 3 years ago, when a
UK court ruling required some websites to block The Pirate Bay. Most
sites in turn used the 403 "forbidden" code, making it unclear to users
about what the issue was. The goal of 451 is to eliminate some of the
confusion around why sites may be blocked.
The use of the code is completely voluntary, however, and requires
developers to begin adopting it. But if widely implemented, it should be
able to communicate to users that some information has been taken down
because of a legal demand, or is being censored by a national
government.