Nowadays it has become very common for Internet providers to be ordered by courts or governments to partially or completely block access to inappropriate contents on websites.The Internet Engineering Task Force (IETF) has approved a special HTTP status code for these type of legal requests, Error 451. This new status code, 451, indicates to the user agent that the host has received a legal request to block access to that particular resource. The response in addition to status code should also include an explanation in the body like details of the legal request, who made it and what legislation has lead to the blockage of the resource. Following is a sample response :

HTTP/1.1 451 Unavailable For Legal Reasons
Content-Type: text/html
Unavailable For Legal Reasons
This request may not be serviced due to.....(details of legal reasons)

Tim Bray had proposed the status code in 2013 and has been approved by the IESG on December 18, 2015.The number 451 refers to Ray Bradburys novel Fahrenheit 451 which focuses on censorship and suppression of information.An example of situation where an HTTP 451 error code could be displayed include web pages containing content deemed to be a danger to national security.