<s>
A	O
Uniform	B-Protocol
Resource	I-Protocol
Name	I-Protocol
(	O
URN	B-Protocol
)	O
is	O
a	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
that	O
uses	O
the	O
scheme	O
.	O
</s>
<s>
URNs	B-Protocol
are	O
globally	O
unique	O
persistent	O
identifiers	O
assigned	O
within	O
defined	O
namespaces	O
so	O
they	O
will	O
be	O
available	O
for	O
a	O
long	O
period	O
of	O
time	O
,	O
even	O
after	O
the	O
resource	O
which	O
they	O
identify	O
ceases	O
to	O
exist	O
or	O
becomes	O
unavailable	O
.	O
</s>
<s>
URNs	B-Protocol
cannot	O
be	O
used	O
to	O
directly	O
locate	O
an	O
item	O
and	O
need	O
not	O
be	O
resolvable	O
,	O
as	O
they	O
are	O
simply	O
templates	O
that	O
another	O
parser	O
may	O
use	O
to	O
find	O
an	O
item	O
.	O
</s>
<s>
URNs	B-Protocol
were	O
originally	O
conceived	O
to	O
be	O
part	O
of	O
a	O
three-part	O
information	B-Architecture
architecture	I-Architecture
for	O
the	O
Internet	O
,	O
along	O
with	O
Uniform	O
Resource	O
Locators	O
(	O
URLs	O
)	O
and	O
Uniform	O
Resource	O
Characteristics	O
(	O
URCs	O
)	O
,	O
a	O
metadata	O
framework	O
.	O
</s>
<s>
As	O
described	O
in	O
RFC	O
1737	O
(	O
1994	O
)	O
,	O
and	O
later	O
in	O
RFC	O
2141	O
(	O
1997	O
)	O
,	O
URNs	B-Protocol
were	O
distinguished	O
from	O
URLs	O
,	O
which	O
identify	O
resources	O
by	O
specifying	O
their	O
locations	O
in	O
the	O
context	O
of	O
a	O
particular	O
access	O
protocol	O
,	O
such	O
as	O
HTTP	B-Protocol
or	O
FTP	B-Protocol
.	O
</s>
<s>
In	O
contrast	O
,	O
URNs	B-Protocol
were	O
conceived	O
as	O
persistent	O
,	O
location-independent	O
identifiers	O
assigned	O
within	O
defined	O
namespaces	O
,	O
typically	O
by	O
an	O
authority	O
responsible	O
for	O
the	O
namespace	O
,	O
so	O
that	O
they	O
are	O
globally	O
unique	O
and	O
persistent	O
over	O
long	O
periods	O
of	O
time	O
,	O
even	O
after	O
the	O
resource	O
which	O
they	O
identify	O
ceases	O
to	O
exist	O
or	O
becomes	O
unavailable	O
.	O
</s>
<s>
Since	O
RFC	O
3986	O
in	O
2005	O
,	O
use	O
of	O
the	O
terms	O
"	O
Uniform	B-Protocol
Resource	I-Protocol
Name	I-Protocol
"	O
and	O
"	O
Uniform	O
Resource	O
Locator	O
"	O
has	O
been	O
deprecated	O
in	O
technical	O
standards	O
in	O
favor	O
of	O
the	O
term	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
,	O
which	O
encompasses	O
both	O
,	O
a	O
view	O
proposed	O
in	O
2001	O
by	O
a	O
joint	O
working	O
group	O
between	O
the	O
World	O
Wide	O
Web	O
Consortium	O
(	O
W3C	O
)	O
and	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
A	O
URI	B-Protocol
is	O
a	O
string	O
of	O
characters	O
used	O
to	O
identify	O
or	O
name	O
a	O
resource	O
on	O
the	O
internet	O
.	O
</s>
<s>
URIs	B-Protocol
are	O
used	O
in	O
many	O
Internet	O
protocols	O
to	O
refer	O
to	O
and	O
access	O
information	O
resources	O
.	O
</s>
<s>
URI	B-Protocol
schemes	O
include	O
the	O
http	B-Protocol
and	O
ftp	B-Protocol
protocols	I-Protocol
,	O
as	O
well	O
as	O
hundreds	O
of	O
others	O
.	O
</s>
<s>
In	O
the	O
"	O
contemporary	O
view	O
"	O
,	O
as	O
it	O
is	O
called	O
,	O
all	O
URIs	B-Protocol
identify	O
or	O
name	O
resources	O
,	O
perhaps	O
uniquely	O
and	O
persistently	O
,	O
with	O
some	O
of	O
them	O
also	O
being	O
"	O
locators	O
"	O
which	O
are	O
resolvable	O
in	O
conjunction	O
with	O
a	O
specified	O
protocol	O
to	O
a	O
representation	O
of	O
the	O
resources	O
.	O
</s>
<s>
Other	O
URIs	B-Protocol
are	O
not	O
locators	O
and	O
are	O
not	O
necessarily	O
resolvable	O
within	O
the	O
bounds	O
of	O
the	O
systems	O
where	O
they	O
are	O
found	O
.	O
</s>
<s>
These	O
URIs	B-Protocol
may	O
serve	O
as	O
names	O
or	O
identifiers	O
of	O
resources	O
.	O
</s>
<s>
But	O
whether	O
a	O
URI	B-Protocol
is	O
resolvable	O
depends	O
on	O
many	O
operational	O
and	O
practical	O
details	O
,	O
irrespective	O
of	O
whether	O
it	O
is	O
called	O
a	O
"	O
name	O
"	O
or	O
a	O
"	O
locator	O
"	O
.	O
</s>
<s>
In	O
accord	O
with	O
this	O
way	O
of	O
thinking	O
,	O
the	O
distinction	O
between	O
Uniform	B-Protocol
Resource	I-Protocol
Names	I-Protocol
and	O
Uniform	O
Resource	O
Locators	O
is	O
now	O
no	O
longer	O
used	O
in	O
formal	O
Internet	O
Engineering	O
Task	O
Force	O
technical	O
standards	O
,	O
though	O
the	O
latter	O
term	O
,	O
URL	O
,	O
is	O
still	O
in	O
wide	O
informal	O
use	O
.	O
</s>
<s>
The	O
term	O
"	O
URN	B-Protocol
"	O
continues	O
now	O
as	O
one	O
of	O
more	O
than	O
a	O
hundred	O
URI	B-Protocol
"	O
schemes	O
"	O
,	O
urn	B-Protocol
:	O
,	O
paralleling	O
http:	O
,	O
ftp	B-Protocol
:	O
,	O
and	O
so	O
forth	O
.	O
</s>
<s>
URIs	B-Protocol
of	O
the	O
urn	B-Protocol
:	O
scheme	O
are	O
not	O
locators	O
,	O
are	O
not	O
required	O
to	O
be	O
associated	O
with	O
a	O
particular	O
protocol	O
or	O
access	O
method	O
,	O
and	O
need	O
not	O
be	O
resolvable	O
.	O
</s>
<s>
Some	O
namespaces	O
under	O
the	O
urn	B-Protocol
:	O
scheme	O
,	O
such	O
as	O
urn:uuid	O
:	O
assign	O
identifiers	O
in	O
a	O
manner	O
which	O
does	O
not	O
require	O
a	O
registration	O
authority	O
,	O
but	O
most	O
of	O
them	O
do	O
.	O
</s>
<s>
A	O
typical	O
URN	B-Protocol
namespace	O
is	O
urn:isbn	B-Library
,	O
for	O
International	B-Library
Standard	I-Library
Book	I-Library
Numbers	I-Library
.	O
</s>
<s>
There	O
are	O
other	O
URI	B-Protocol
schemes	O
,	O
such	O
as	O
tag	O
:	O
,	O
info	B-Protocol
:	I-Protocol
(	O
now	O
largely	O
deprecated	O
)	O
,	O
and	O
ni	O
:	O
which	O
are	O
similar	O
to	O
the	O
urn	B-Protocol
:	O
scheme	O
in	O
not	O
being	O
locators	O
and	O
not	O
being	O
associated	O
with	O
particular	O
resolution	O
or	O
access	O
protocols	O
.	O
</s>
<s>
The	O
syntax	O
of	O
a	O
urn	B-Protocol
:	O
scheme	O
URI	B-Protocol
is	O
represented	O
in	O
the	O
augmented	O
Backus	O
–	O
Naur	O
form	O
as	O
:	O
</s>
<s>
or	O
,	O
in	O
the	O
form	O
of	O
a	O
syntax	B-Application
diagram	I-Application
,	O
as	O
:	O
</s>
<s>
The	O
NSS	O
may	O
contain	O
ASCII	B-Protocol
letters	I-Protocol
and	O
digits	O
,	O
and	O
many	O
punctuation	O
and	O
special	O
characters	O
.	O
</s>
<s>
Disallowed	O
ASCII	B-Protocol
and	O
Unicode	O
characters	O
may	O
be	O
included	O
if	O
percent-encoded	O
.	O
</s>
<s>
In	O
2017	O
,	O
the	O
syntax	O
for	O
URNs	B-Protocol
was	O
updated	O
:	O
</s>
<s>
The	O
slash	O
character	O
(	O
/	O
)	O
is	O
now	O
allowed	O
in	O
the	O
NSS	O
to	O
represent	O
names	O
containing	O
slashes	O
from	O
non-URN	O
identifier	O
systems	O
.	O
</s>
<s>
In	O
order	O
to	O
ensure	O
the	O
global	O
uniqueness	O
of	O
URN	B-Protocol
namespaces	O
,	O
their	O
identifiers	O
(	O
NIDs	O
)	O
are	O
required	O
to	O
be	O
registered	O
with	O
the	O
IANA	O
.	O
</s>
<s>
Approximately	O
sixty	O
formal	O
URN	B-Protocol
namespace	O
identifiers	O
have	O
been	O
registered	O
.	O
</s>
<s>
Informal	O
namespaces	O
are	O
fully	O
fledged	O
URN	B-Protocol
namespaces	O
and	O
can	O
be	O
registered	O
in	O
global	O
registration	O
services	O
.	O
</s>
<s>
However	O
,	O
following	O
the	O
deprecation	O
of	O
the	O
"	O
X-	O
"	O
notation	O
for	O
new	O
identifier	O
names	O
,	O
RFC	O
8141	O
did	O
away	O
with	O
experimental	O
URN	B-Protocol
namespaces	O
,	O
indicating	O
a	O
preference	O
for	O
use	O
of	O
the	O
urn:example	O
namespace	O
where	O
appropriate	O
.	O
</s>
<s>
URN	B-Protocol
corresponds	O
to	O
The	O
1968	O
book	O
The	O
Last	O
Unicorn	O
,	O
identified	O
by	O
its	O
International	B-Library
Standard	I-Library
Book	I-Library
Number	I-Library
.	O
</s>
<s>
A	O
directive	O
of	O
the	O
European	O
Union	O
,	O
using	O
the	O
proposed	O
Lex	O
URN	B-Protocol
namespace	O
.	O
</s>
<s>
A	O
Life	O
Science	O
Identifiers	O
that	O
may	O
be	O
resolved	O
to	O
http://zoobank.org/urn:lsid:zoobank.org:pub:CDC8D258-8F57-41DC-B560-247E17D3DC8C	O
.	O
</s>
