<s>
A	O
Uniform	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
URI	B-Protocol
)	O
is	O
a	O
unique	O
sequence	O
of	O
characters	O
that	O
identifies	O
a	O
logical	O
or	O
physical	O
resource	O
used	O
by	O
web	O
technologies	O
.	O
</s>
<s>
URIs	B-Protocol
may	O
be	O
used	O
to	O
identify	O
anything	O
,	O
including	O
real-world	O
objects	O
,	O
such	O
as	O
people	O
and	O
places	O
,	O
concepts	O
,	O
or	O
information	O
resources	O
such	O
as	O
web	O
pages	O
and	O
books	O
.	O
</s>
<s>
Some	O
URIs	B-Protocol
provide	O
a	O
means	O
of	O
locating	O
and	O
retrieving	O
information	O
resources	O
on	O
a	O
network	O
(	O
either	O
on	O
the	O
Internet	O
or	O
on	O
another	O
private	O
network	O
,	O
such	O
as	O
a	O
computer	O
filesystem	O
or	O
an	O
Intranet	B-General_Concept
)	O
;	O
these	O
are	O
Uniform	O
Resource	O
Locators	O
(	O
URLs	O
)	O
.	O
</s>
<s>
A	O
URI	B-Protocol
identifies	O
the	O
resource	O
by	O
name	O
at	O
the	O
specified	O
location	O
or	O
URL	O
.	O
</s>
<s>
Other	O
URIs	B-Protocol
provide	O
only	O
a	O
unique	O
name	O
,	O
without	O
a	O
means	O
of	O
locating	O
or	O
retrieving	O
the	O
resource	O
or	O
information	O
about	O
it	O
,	O
these	O
are	O
Uniform	B-Protocol
Resource	I-Protocol
Names	I-Protocol
(	O
URNs	B-Protocol
)	O
.	O
</s>
<s>
The	O
web	O
technologies	O
that	O
use	O
URIs	B-Protocol
are	O
not	O
limited	O
to	O
web	O
browsers	O
.	O
</s>
<s>
URIs	B-Protocol
are	O
used	O
to	O
identify	O
anything	O
described	O
using	O
the	O
Resource	O
Description	O
Framework	O
(	O
RDF	O
)	O
,	O
for	O
example	O
,	O
concepts	O
that	O
are	O
part	O
of	O
an	O
ontology	O
defined	O
using	O
the	O
Web	B-Language
Ontology	I-Language
Language	I-Language
(	O
OWL	B-Language
)	O
,	O
and	O
people	O
who	O
are	O
described	O
using	O
the	O
Friend	O
of	O
a	O
Friend	O
vocabulary	O
would	O
each	O
have	O
an	O
individual	O
URI	B-Protocol
.	O
</s>
<s>
URIs	B-Protocol
and	O
URLs	O
have	O
a	O
shared	O
history	O
.	O
</s>
<s>
Over	O
the	O
next	O
three	O
and	O
a	O
half	O
years	O
,	O
as	O
the	O
World	O
Wide	O
Web	O
's	O
core	O
technologies	O
of	O
HTML	B-Language
,	O
HTTP	B-Protocol
,	O
and	O
web	O
browsers	O
developed	O
,	O
a	O
need	O
to	O
distinguish	O
a	O
string	O
that	O
provided	O
an	O
address	O
for	O
a	O
resource	O
from	O
a	O
string	O
that	O
merely	O
named	O
a	O
resource	O
emerged	O
.	O
</s>
<s>
Although	O
not	O
yet	O
formally	O
defined	O
,	O
the	O
term	O
Uniform	O
Resource	O
Locator	O
came	O
to	O
represent	O
the	O
former	O
,	O
and	O
the	O
more	O
contentious	O
Uniform	B-Protocol
Resource	I-Protocol
Name	I-Protocol
came	O
to	O
represent	O
the	O
latter	O
.	O
</s>
<s>
In	O
July	O
1992	O
Berners-Lee	O
'	O
s	O
report	O
on	O
the	O
IETF	O
"	O
UDI	O
(	O
Universal	O
Document	O
Identifiers	O
)	O
BOF	O
"	O
mentions	O
URLs	O
(	O
as	O
Uniform	O
Resource	O
Locators	O
)	O
,	O
URNs	B-Protocol
(	O
originally	O
,	O
as	O
Unique	O
Resource	O
Numbers	O
)	O
,	O
and	O
the	O
need	O
to	O
charter	O
a	O
new	O
working	O
group	O
.	O
</s>
<s>
In	O
November	O
1992	O
the	O
IETF	O
"	O
URI	B-Protocol
Working	O
Group	O
"	O
met	O
for	O
the	O
first	O
time	O
.	O
</s>
<s>
During	O
the	O
debate	O
over	O
defining	O
URLs	O
and	O
URNs	B-Protocol
,	O
it	O
became	O
evident	O
that	O
the	O
concepts	O
embodied	O
by	O
the	O
two	O
terms	O
were	O
merely	O
aspects	O
of	O
the	O
fundamental	O
,	O
overarching	O
,	O
notion	O
of	O
resource	O
identification	O
.	O
</s>
<s>
In	O
June	O
1994	O
,	O
the	O
IETF	O
published	O
Berners-Lee	O
'	O
s	O
first	O
Request	O
for	O
Comments	O
that	O
acknowledged	O
the	O
existence	O
of	O
URLs	O
and	O
URNs	B-Protocol
.	O
</s>
<s>
Most	O
importantly	O
,	O
it	O
defined	O
a	O
formal	O
syntax	O
for	O
Universal	B-Protocol
Resource	I-Protocol
Identifiers	I-Protocol
(	O
i.e.	O
</s>
<s>
The	O
agreed	O
definition	O
and	O
syntax	O
of	O
URNs	B-Protocol
had	O
to	O
wait	O
until	O
the	O
publication	O
of	O
IETF	O
RFC	O
2141	O
in	O
May	O
1997	O
.	O
</s>
<s>
The	O
publication	O
of	O
IETF	O
RFC	O
2396	O
in	O
August	O
1998	O
saw	O
the	O
URI	B-Protocol
syntax	O
become	O
a	O
separate	O
specification	O
and	O
most	O
of	O
the	O
parts	O
of	O
RFCs	O
1630	O
and	O
1738	O
relating	O
to	O
URIs	B-Protocol
and	O
URLs	O
in	O
general	O
were	O
revised	O
and	O
expanded	O
by	O
the	O
IETF	O
.	O
</s>
<s>
The	O
new	O
RFC	O
changed	O
the	O
meaning	O
of	O
"	O
U	O
"	O
in	O
"	O
URI	B-Protocol
"	O
to	O
"	O
Uniform	O
"	O
from	O
"	O
Universal	O
"	O
.	O
</s>
<s>
In	O
December	O
1999	O
,	O
provided	O
a	O
minor	O
update	O
to	O
RFC	O
2396	O
,	O
allowing	O
URIs	B-Protocol
to	O
accommodate	O
IPv6	B-Protocol
addresses	O
.	O
</s>
<s>
IETF	O
RFC	B-Protocol
2616	I-Protocol
for	O
example	O
,	O
refines	O
the	O
http	B-Protocol
scheme	O
.	O
</s>
<s>
Simultaneously	O
,	O
the	O
IETF	O
published	O
the	O
content	O
of	O
RFC	O
3986	O
as	O
the	O
full	O
standard	O
STD	O
66	O
,	O
reflecting	O
the	O
establishment	O
of	O
the	O
URI	B-Protocol
generic	O
syntax	O
as	O
an	O
official	O
Internet	O
protocol	O
.	O
</s>
<s>
In	O
2001	O
,	O
the	O
W3C	O
's	O
Technical	O
Architecture	O
Group	O
(	O
TAG	O
)	O
published	O
a	O
guide	O
to	O
best	O
practices	O
and	O
canonical	O
URIs	B-Protocol
for	O
publishing	O
multiple	O
versions	O
of	O
a	O
given	O
resource	O
.	O
</s>
<s>
In	O
August	O
2002	O
,	O
IETF	O
pointed	O
out	O
that	O
the	O
term	O
"	O
URL	O
"	O
had	O
,	O
despite	O
widespread	O
public	O
use	O
,	O
faded	O
into	O
near	O
obsolescence	O
,	O
and	O
serves	O
only	O
as	O
a	O
reminder	O
that	O
some	O
URIs	B-Protocol
act	O
as	O
addresses	O
by	O
having	O
schemes	O
implying	O
network	O
accessibility	O
,	O
regardless	O
of	O
any	O
such	O
actual	O
use	O
.	O
</s>
<s>
As	O
URI-based	O
standards	O
such	O
as	O
Resource	O
Description	O
Framework	O
make	O
evident	O
,	O
resource	O
identification	O
need	O
not	O
suggest	O
the	O
retrieval	O
of	O
resource	O
representations	O
over	O
the	O
Internet	O
,	O
nor	O
need	O
they	O
imply	O
network-based	O
resources	O
at	O
all	O
.	O
</s>
<s>
The	O
Semantic	O
Web	O
uses	O
the	O
HTTP	B-Protocol
URI	B-Protocol
scheme	O
to	O
identify	O
both	O
documents	O
and	O
concepts	O
in	O
the	O
real	O
world	O
,	O
a	O
distinction	O
which	O
has	O
caused	O
confusion	O
as	O
to	O
how	O
to	O
distinguish	O
the	O
two	O
.	O
</s>
<s>
The	O
W3C	O
subsequently	O
published	O
an	O
Interest	O
Group	O
Note	O
titled	O
Cool	O
URIs	B-Protocol
for	O
the	O
Semantic	O
Web	O
,	O
which	O
explained	O
the	O
use	O
of	O
content	B-Protocol
negotiation	I-Protocol
and	O
the	O
HTTP	B-Protocol
303	I-Protocol
response	O
code	O
for	O
redirections	O
in	O
more	O
detail	O
.	O
</s>
<s>
A	O
Uniform	B-Protocol
Resource	I-Protocol
Name	I-Protocol
(	O
URN	B-Protocol
)	O
is	O
a	O
URI	B-Protocol
that	O
identifies	O
a	O
resource	O
by	O
name	O
in	O
a	O
particular	O
namespace	B-Language
.	O
</s>
<s>
A	O
URN	B-Protocol
may	O
be	O
used	O
to	O
talk	O
about	O
a	O
resource	O
without	O
implying	O
its	O
location	O
or	O
how	O
to	O
access	O
it	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
International	B-Library
Standard	I-Library
Book	I-Library
Number	I-Library
(	O
ISBN	B-Library
)	O
system	O
,	O
ISBN	B-Library
0-486-27557-4	O
identifies	O
a	O
specific	O
edition	O
of	O
Shakespeare	O
's	O
play	O
Romeo	O
and	O
Juliet	O
.	O
</s>
<s>
The	O
URN	B-Protocol
for	O
that	O
edition	O
would	O
be	O
urn:isbn:0-486-27557-4	O
.	O
</s>
<s>
A	O
Uniform	O
Resource	O
Locator	O
(	O
URL	O
)	O
is	O
a	O
URI	B-Protocol
that	O
specifies	O
the	O
means	O
of	O
acting	O
upon	O
or	O
obtaining	O
the	O
representation	O
of	O
a	O
resource	O
,	O
i.e.	O
</s>
<s>
For	O
example	O
,	O
the	O
URL	O
http://example.org/wiki/Main_Page	O
refers	O
to	O
a	O
resource	O
identified	O
as	O
/wiki/Main_Page	O
,	O
whose	O
representation	O
is	O
obtainable	O
via	O
the	O
Hypertext	B-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
http	B-Protocol
:	I-Protocol
)	O
from	O
a	O
network	O
host	O
whose	O
domain	O
name	O
is	O
example.org	O
.	O
</s>
<s>
(	O
In	O
this	O
case	O
,	O
HTTP	B-Protocol
usually	O
implies	O
it	O
to	O
be	O
in	O
the	O
form	O
of	O
HTML	B-Language
and	O
related	O
code	O
.	O
</s>
<s>
In	O
practice	O
,	O
that	O
is	O
not	O
necessarily	O
the	O
case	O
,	O
as	O
HTTP	B-Protocol
allows	O
specifying	O
arbitrary	O
formats	O
in	O
its	O
header	O
.	O
)	O
</s>
<s>
A	O
URN	B-Protocol
is	O
analogous	O
to	O
a	O
person	O
's	O
name	O
,	O
while	O
a	O
URL	O
is	O
analogous	O
to	O
their	O
street	O
address	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
a	O
URN	B-Protocol
identifies	O
an	O
item	O
and	O
a	O
URL	O
provides	O
a	O
method	O
for	O
finding	O
it	O
.	O
</s>
<s>
Technical	O
publications	O
,	O
especially	O
standards	O
produced	O
by	O
the	O
IETF	O
and	O
by	O
the	O
W3C	O
,	O
normally	O
reflect	O
a	O
view	O
outlined	O
in	O
a	O
W3C	O
Recommendation	O
of	O
30	O
July	O
2001	O
,	O
which	O
acknowledges	O
the	O
precedence	O
of	O
the	O
term	O
URI	B-Protocol
rather	O
than	O
endorsing	O
any	O
formal	O
subdivision	O
into	O
URL	O
and	O
URN	B-Protocol
.	O
</s>
<s>
As	O
such	O
,	O
a	O
URL	O
is	O
simply	O
a	O
URI	B-Protocol
that	O
happens	O
to	O
point	O
to	O
a	O
resource	O
over	O
a	O
network	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
term	O
"	O
web	O
address	O
"	O
(	O
which	O
has	O
no	O
formal	O
definition	O
)	O
often	O
occurs	O
in	O
non-technical	O
publications	O
as	O
a	O
synonym	O
for	O
a	O
URI	B-Protocol
that	O
uses	O
the	O
http	B-Protocol
or	O
https	B-Protocol
schemes	I-Protocol
.	O
</s>
<s>
Such	O
assumptions	O
can	O
lead	O
to	O
confusion	O
,	O
for	O
example	O
,	O
in	O
the	O
case	O
of	O
XML	B-Language
namespaces	I-Language
that	O
have	O
a	O
visual	O
similarity	O
to	O
resolvable	O
URIs	B-Protocol
.	O
</s>
<s>
Specifications	O
produced	O
by	O
the	O
WHATWG	B-Language
prefer	O
URL	O
over	O
URI	B-Protocol
,	O
and	O
so	O
newer	O
HTML5	O
APIs	O
use	O
URL	O
over	O
URI	B-Protocol
.	O
</s>
<s>
While	O
most	O
URI	B-Protocol
schemes	O
were	O
originally	O
designed	O
to	O
be	O
used	O
with	O
a	O
particular	O
protocol	O
,	O
and	O
often	O
have	O
the	O
same	O
name	O
,	O
they	O
are	O
semantically	O
different	O
from	O
protocols	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
scheme	O
http	B-Protocol
is	O
generally	O
used	O
for	O
interacting	O
with	O
web	O
resources	O
using	O
HTTP	B-Protocol
,	O
but	O
the	O
scheme	O
file	O
has	O
no	O
protocol	O
.	O
</s>
<s>
A	O
URI	B-Protocol
has	O
a	O
scheme	O
that	O
refers	O
to	O
a	O
specification	O
for	O
assigning	O
identifiers	O
within	O
that	O
scheme	O
.	O
</s>
<s>
As	O
such	O
,	O
the	O
URI	B-Protocol
syntax	O
is	O
a	O
federated	O
and	O
extensible	O
naming	O
system	O
wherein	O
each	O
scheme	O
's	O
specification	O
may	O
further	O
restrict	O
the	O
syntax	O
and	O
semantics	O
of	O
identifiers	O
using	O
that	O
scheme	O
.	O
</s>
<s>
The	O
URI	B-Protocol
generic	O
syntax	O
is	O
a	O
superset	O
of	O
the	O
syntax	O
of	O
all	O
URI	B-Protocol
schemes	O
.	O
</s>
<s>
A	O
URI	B-Protocol
is	O
composed	O
from	O
an	O
allowed	O
set	O
of	O
ASCII	B-Protocol
characters	I-Protocol
consisting	O
of	O
reserved	O
characters	O
(	O
generic	O
:	O
:	O
,	O
/	O
,	O
?,	O
#	O
,	O
[	O
,	O
]	O
,	O
and	O
@	O
;	O
scheme	O
-	O
or	O
implementation-specific	O
:	O
!,	O
$	O
,	O
&	O
,	O
'	O
,	O
(	O
,	O
)	O
,	O
*	O
,	O
+	O
,	O
,,	O
;	O
,	O
and	O
=	O
)	O
,	O
unreserved	O
characters	O
(	O
uppercase	O
and	O
lowercase	O
letters	O
,	O
decimal	O
digits	O
,	O
-	O
,	O
.	O
,	O
_	O
,	O
and	O
~	O
)	O
,	O
and	O
the	O
character	O
%	O
.	O
</s>
<s>
Syntax	O
components	O
and	O
subcomponents	O
are	O
separated	O
by	O
delimiters	B-Algorithm
from	O
the	O
reserved	O
characters	O
(	O
only	O
from	O
generic	O
reserved	O
characters	O
for	O
components	O
)	O
and	O
define	O
identifying	O
data	B-Protocol
represented	O
as	O
unreserved	O
characters	O
,	O
reserved	O
characters	O
that	O
do	O
not	O
act	O
as	O
delimiters	B-Algorithm
in	O
the	O
component	O
and	O
subcomponent	O
respectively	O
,	O
and	O
percent-encodings	O
when	O
the	O
corresponding	O
character	O
is	O
outside	O
the	O
allowed	O
set	O
or	O
is	O
being	O
used	O
as	O
a	O
delimiter	B-Algorithm
of	O
,	O
or	O
within	O
,	O
the	O
component	O
.	O
</s>
<s>
A	O
percent-encoding	O
of	O
an	O
identifying	O
data	B-Protocol
octet	O
is	O
a	O
sequence	O
of	O
three	O
characters	O
,	O
consisting	O
of	O
the	O
character	O
%	O
followed	O
by	O
the	O
two	O
hexadecimal	O
digits	O
representing	O
that	O
octet	O
's	O
numeric	O
value	O
.	O
</s>
<s>
The	O
URI	B-Protocol
generic	O
syntax	O
consists	O
of	O
five	O
components	O
organized	O
hierarchically	O
in	O
order	O
of	O
decreasing	O
significance	O
from	O
left	O
to	O
right	O
:	O
</s>
<s>
A	O
component	O
is	O
undefined	O
if	O
it	O
has	O
an	O
associated	O
delimiter	B-Algorithm
and	O
the	O
delimiter	B-Algorithm
does	O
not	O
appear	O
in	O
the	O
URI	B-Protocol
;	O
the	O
scheme	O
and	O
path	O
components	O
are	O
always	O
defined	O
.	O
</s>
<s>
This	O
is	O
represented	O
in	O
a	O
syntax	B-Application
diagram	I-Application
as	O
:	O
</s>
<s>
The	O
URI	B-Protocol
comprises	O
:	O
</s>
<s>
Examples	O
of	O
popular	O
schemes	O
include	O
http	B-Protocol
,	O
https	B-Protocol
,	O
ftp	B-Protocol
,	O
mailto	O
,	O
file	O
,	O
data	B-Protocol
and	O
irc	O
.	O
</s>
<s>
URI	B-Protocol
schemes	O
should	O
be	O
registered	O
with	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
,	O
although	O
non-registered	O
schemes	O
are	O
used	O
in	O
practice	O
.	O
</s>
<s>
Applications	O
should	O
not	O
render	O
as	O
clear	O
text	O
any	O
data	B-Protocol
after	O
the	O
first	O
colon	O
(	O
:	O
)	O
found	O
within	O
a	O
userinfo	O
subcomponent	O
unless	O
the	O
data	B-Protocol
after	O
the	O
colon	O
is	O
the	O
empty	O
string	O
(	O
indicating	O
no	O
password	O
)	O
.	O
</s>
<s>
A	O
subcomponent	O
,	O
consisting	O
of	O
either	O
a	O
registered	O
name	O
(	O
including	O
but	O
not	O
limited	O
to	O
a	O
hostname	B-Protocol
)	O
or	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
IPv4	B-Protocol
addresses	O
must	O
be	O
in	O
dot-decimal	B-Protocol
notation	I-Protocol
,	O
and	O
IPv6	B-Protocol
addresses	O
must	O
be	O
enclosed	O
in	O
brackets	O
( []	O
)	O
.	O
</s>
<s>
A	O
path	O
is	O
always	O
defined	O
for	O
a	O
URI	B-Protocol
,	O
though	O
the	O
defined	O
path	O
may	O
be	O
empty	O
(	O
zero	O
length	O
)	O
.	O
</s>
<s>
A	O
path	O
component	O
may	O
resemble	O
or	O
map	O
exactly	O
to	O
a	O
file	B-Application
system	I-Application
path	I-Application
but	O
does	O
not	O
always	O
imply	O
a	O
relation	O
to	O
one	O
.	O
</s>
<s>
By	O
convention	O
,	O
in	O
http	B-Protocol
and	O
https	B-Protocol
URIs	I-Protocol
,	O
the	O
last	O
part	O
of	O
a	O
path	O
is	O
named	O
and	O
it	O
is	O
optional	O
.	O
</s>
<s>
a	O
command	O
or	O
a	O
qualifier	O
part	O
)	O
that	O
has	O
to	O
be	O
passed	O
separately	O
to	O
the	O
first	O
part	O
of	O
the	O
path	O
that	O
identifies	O
an	O
executable	O
module	O
or	O
program	O
managed	O
by	O
a	O
web	B-Application
server	I-Application
;	O
this	O
is	O
often	O
used	O
to	O
select	O
dynamic	O
content	O
(	O
a	O
document	O
,	O
etc	O
.	O
)	O
</s>
<s>
or	O
to	O
tailor	O
it	O
as	O
requested	O
(	O
see	O
also	O
:	O
CGI	B-Language
and	O
PATH_INFO	O
,	O
etc	O
.	O
</s>
<s>
URI	B-Protocol
:	O
</s>
<s>
An	O
http	B-Protocol
or	O
https	B-Protocol
URI	I-Protocol
containing	O
a	O
pathinfo	O
part	O
without	O
a	O
query	O
part	O
may	O
also	O
be	O
referred	O
to	O
as	O
a	O
'	O
clean	B-Protocol
URL	I-Protocol
 '	O
whose	O
last	O
part	O
may	O
be	O
a	O
'	O
slug	O
 '	O
.	O
</s>
<s>
)	O
,	O
consisting	O
of	O
a	O
query	B-Protocol
string	I-Protocol
of	O
non-hierarchical	O
data	B-Protocol
.	O
</s>
<s>
Its	O
syntax	O
is	O
not	O
well	O
defined	O
,	O
but	O
by	O
convention	O
is	O
most	O
often	O
a	O
sequence	O
of	O
attribute	B-Application
–	I-Application
value	I-Application
pairs	I-Application
separated	O
by	O
a	O
delimiter	B-Algorithm
.	O
</s>
<s>
The	O
fragment	O
contains	O
a	O
fragment	O
identifier	O
providing	O
direction	O
to	O
a	O
secondary	O
resource	O
,	O
such	O
as	O
a	O
section	O
heading	O
in	O
an	O
article	O
identified	O
by	O
the	O
remainder	O
of	O
the	O
URI	B-Protocol
.	O
</s>
<s>
When	O
the	O
primary	O
resource	O
is	O
an	O
HTML	B-Language
document	O
,	O
the	O
fragment	O
is	O
often	O
an	O
id	O
attribute	O
of	O
a	O
specific	O
element	O
,	O
and	O
web	O
browsers	O
will	O
scroll	O
this	O
element	O
into	O
view	O
.	O
</s>
<s>
The	O
following	O
figure	O
displays	O
example	O
URIs	B-Protocol
and	O
their	O
component	O
parts	O
.	O
</s>
<s>
DOIs	O
(	O
digital	O
object	O
identifiers	O
)	O
fit	O
within	O
the	O
Handle	O
System	O
and	O
fit	O
within	O
the	O
URI	B-Protocol
system	O
,	O
as	O
facilitated	O
by	O
appropriate	O
syntax	O
.	O
</s>
<s>
A	O
URI	B-Protocol
reference	O
is	O
either	O
a	O
URI	B-Protocol
or	O
a	O
relative	O
reference	O
when	O
it	O
does	O
not	O
begin	O
with	O
a	O
scheme	O
component	O
followed	O
by	O
a	O
colon	O
(	O
:	O
)	O
.	O
</s>
<s>
Web	O
document	O
markup	O
languages	O
frequently	O
use	O
URI	B-Protocol
references	O
to	O
point	O
to	O
other	O
resources	O
,	O
such	O
as	O
external	O
documents	O
or	O
specific	O
portions	O
of	O
the	O
same	O
logical	O
document	O
:	O
</s>
<s>
in	O
HTML	B-Language
,	O
the	O
value	O
of	O
the	O
src	O
attribute	O
of	O
the	O
img	O
element	O
provides	O
a	O
URI	B-Protocol
reference	O
,	O
as	O
does	O
the	O
value	O
of	O
the	O
href	O
attribute	O
of	O
the	O
a	O
or	O
link	O
element	O
;	O
</s>
<s>
in	O
XML	B-Protocol
,	O
the	O
system	B-Language
identifier	I-Language
appearing	O
after	O
the	O
SYSTEM	O
keyword	O
in	O
a	O
DTD	B-Language
is	O
a	O
fragmentless	O
URI	B-Protocol
reference	O
;	O
</s>
<s>
in	O
XSLT	B-Application
,	O
the	O
value	O
of	O
the	O
href	O
attribute	O
of	O
the	O
xsl:import	O
element/instruction	O
is	O
a	O
URI	B-Protocol
reference	O
;	O
likewise	O
the	O
first	O
argument	O
to	O
the	O
document( )	O
function	O
.	O
</s>
<s>
Resolving	O
a	O
URI	B-Protocol
reference	O
against	O
a	O
base	O
URI	B-Protocol
results	O
in	O
a	O
target	O
URI	B-Protocol
.	O
</s>
<s>
This	O
implies	O
that	O
the	O
base	O
URI	B-Protocol
exists	O
and	O
is	O
an	O
absolute	O
URI	B-Protocol
(	O
a	O
URI	B-Protocol
with	O
no	O
fragment	O
component	O
)	O
.	O
</s>
<s>
The	O
base	O
URI	B-Protocol
can	O
be	O
obtained	O
,	O
in	O
order	O
of	O
precedence	O
,	O
from	O
:	O
</s>
<s>
the	O
reference	O
URI	B-Protocol
itself	O
if	O
it	O
is	O
a	O
URI	B-Protocol
;	O
</s>
<s>
the	O
URI	B-Protocol
used	O
for	O
the	O
actual	O
retrieval	O
of	O
the	O
representation	O
;	O
</s>
<s>
a	O
relative	O
reference	O
is	O
resolved	O
to	O
its	O
target	O
URI	B-Protocol
as	O
follows	O
:	O
</s>
<s>
It	O
's	O
commonly	O
used	O
in	O
WebDAV	B-Protocol
as	O
a	O
mechanism	O
of	O
adding	O
functionality	O
to	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
It	O
has	O
the	O
advantage	O
of	O
both	O
being	O
easy	O
for	O
CGI	B-Language
parsers	O
and	O
also	O
acts	O
as	O
an	O
intermediary	O
between	O
HTTP	B-Protocol
and	O
underlying	O
resource	O
,	O
in	O
this	O
case	O
.	O
</s>
<s>
In	O
XML	B-Protocol
,	O
a	O
namespace	B-Language
is	O
an	O
abstract	O
domain	O
to	O
which	O
a	O
collection	O
of	O
element	O
and	O
attribute	O
names	O
can	O
be	O
assigned	O
.	O
</s>
<s>
The	O
namespace	B-Language
name	O
is	O
a	O
character	O
string	O
which	O
must	O
adhere	O
to	O
the	O
generic	O
URI	B-Protocol
syntax	O
.	O
</s>
<s>
However	O
,	O
the	O
name	O
is	O
generally	O
not	O
considered	O
to	O
be	O
a	O
URI	B-Protocol
,	O
because	O
the	O
URI	B-Protocol
specification	O
bases	O
the	O
decision	O
not	O
only	O
on	O
lexical	O
components	O
,	O
but	O
also	O
on	O
their	O
intended	O
use	O
.	O
</s>
<s>
A	O
namespace	B-Language
name	O
does	O
not	O
necessarily	O
imply	O
any	O
of	O
the	O
semantics	O
of	O
URI	B-Protocol
schemes	O
;	O
for	O
example	O
,	O
a	O
namespace	B-Language
name	O
beginning	O
with	O
http:	O
may	O
have	O
no	O
connotation	O
to	O
the	O
use	O
of	O
the	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
Originally	O
,	O
the	O
namespace	B-Language
name	O
could	O
match	O
the	O
syntax	O
of	O
any	O
non-empty	O
URI	B-Protocol
reference	O
,	O
but	O
the	O
use	O
of	O
relative	O
URI	B-Protocol
references	O
was	O
deprecated	O
by	O
the	O
W3C	O
.	O
</s>
<s>
A	O
separate	O
W3C	O
specification	O
for	O
namespaces	B-Language
in	I-Language
XML	I-Language
1.1	O
permits	O
Internationalized	B-Protocol
Resource	I-Protocol
Identifier	I-Protocol
(	O
IRI	O
)	O
references	O
to	O
serve	O
as	O
the	O
basis	O
for	O
namespace	B-Language
names	O
in	O
addition	O
to	O
URI	B-Protocol
references	O
.	O
</s>
