<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
HTTP	B-Protocol
451	I-Protocol
Unavailable	B-Protocol
For	I-Protocol
Legal	I-Protocol
Reasons	I-Protocol
is	O
a	O
proposed	O
standard	O
error	O
status	B-Protocol
code	I-Protocol
of	O
the	O
HTTP	B-Protocol
protocol	I-Protocol
to	O
be	O
displayed	O
when	O
the	O
user	O
requests	O
a	O
resource	O
which	O
cannot	O
be	O
served	O
for	O
legal	O
reasons	O
,	O
such	O
as	O
a	O
web	O
page	O
censored	O
by	O
a	O
government	O
.	O
</s>
<s>
The	O
number	O
451	O
is	O
a	O
reference	O
to	O
Ray	O
Bradbury	O
's	O
1953	O
dystopian	O
novel	O
Fahrenheit	B-Device
451	I-Device
,	O
in	O
which	O
books	O
are	O
outlawed	O
.	O
</s>
<s>
451	O
provides	O
more	O
information	O
than	O
HTTP	B-Protocol
403	I-Protocol
,	O
which	O
is	O
often	O
used	O
for	O
the	O
same	O
purpose	O
.	O
</s>
<s>
This	O
status	B-Protocol
code	I-Protocol
is	O
currently	O
a	O
proposed	O
standard	O
in	O
but	O
is	O
not	O
yet	O
formally	O
a	O
part	O
of	O
HTTP	B-Protocol
,	O
as	O
of	O
.	O
</s>
<s>
Examples	O
of	O
situations	O
where	O
an	O
HTTP	B-Protocol
451	I-Protocol
error	O
code	O
could	O
be	O
displayed	O
include	O
web	O
pages	O
deemed	O
a	O
danger	O
to	O
national	O
security	O
,	O
or	O
web	O
pages	O
deemed	O
to	O
violate	O
copyright	O
,	O
privacy	O
,	O
blasphemy	O
laws	O
,	O
or	O
any	O
other	O
law	O
or	O
court	O
order	O
.	O
</s>
<s>
Some	O
sites	O
have	O
previously	O
returned	O
HTTP	B-Protocol
404	I-Protocol
(	O
missing	O
)	O
or	O
similar	O
if	O
they	O
are	O
not	O
legally	O
permitted	O
to	O
disclose	O
that	O
the	O
resource	O
has	O
been	O
removed	O
.	O
</s>
<s>
It	O
is	O
used	O
in	O
the	O
United	O
Kingdom	O
by	O
some	O
Internet	O
service	O
providers	O
utilising	O
the	O
Internet	O
Watch	O
Foundation	O
blacklist	O
,	O
returning	O
a	O
404	B-Protocol
message	O
or	O
another	O
error	O
message	O
instead	O
of	O
showing	O
a	O
message	O
indicating	O
the	O
site	O
is	O
blocked	O
.	O
</s>
<s>
The	O
status	B-Protocol
code	I-Protocol
was	O
formally	O
proposed	O
in	O
2013	O
by	O
Tim	O
Bray	O
,	O
following	O
earlier	O
informal	O
proposals	O
by	O
Chris	O
Applegate	O
in	O
2008	O
and	O
Terence	O
Eden	O
in	O
2012	O
.	O
</s>
<s>
HTTP	B-Protocol
451	I-Protocol
was	O
mentioned	O
by	O
the	O
BBC	O
's	O
From	O
Our	O
Own	O
Correspondent	O
programme	O
,	O
as	O
an	O
indication	O
of	O
the	O
effects	O
of	O
sanctions	O
on	O
Sudan	O
and	O
the	O
inability	O
to	O
access	O
Airbnb	O
,	O
the	O
App	B-Application
Store	I-Application
,	O
or	O
other	O
Western	O
web	O
services	O
.	O
</s>
<s>
After	O
introduction	O
of	O
the	O
GDPR	O
in	O
EEA	O
it	O
became	O
common	O
practice	O
for	O
websites	O
located	O
outside	O
EEA	O
to	O
serve	O
HTTP	B-Protocol
451	I-Protocol
errors	O
to	O
EEA	O
visitors	O
instead	O
of	O
trying	O
to	O
comply	O
with	O
this	O
new	O
privacy	O
law	O
.	O
</s>
<s>
When	O
an	O
entity	O
intercepts	O
the	O
request	O
and	O
returns	O
status	O
451	O
,	O
it	O
should	O
include	O
a	O
"	O
Link	O
"	O
HTTP	B-Protocol
header	O
field	O
whose	O
value	O
is	O
a	O
URI	O
reference	O
identifying	O
itself	O
.	O
</s>
