<s>
A	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
(	O
FQDN	B-Protocol
)	O
,	O
sometimes	O
also	O
referred	O
to	O
as	O
an	O
absolute	O
domain	O
name	O
,	O
is	O
a	O
domain	O
name	O
that	O
specifies	O
its	O
exact	O
location	O
in	O
the	O
tree	O
hierarchy	O
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
.	O
</s>
<s>
It	O
specifies	O
all	O
domain	O
levels	O
,	O
including	O
the	O
top-level	O
domain	O
and	O
the	O
root	B-Protocol
zone	I-Protocol
.	O
</s>
<s>
A	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
is	O
distinguished	O
by	O
its	O
lack	O
of	O
ambiguity	O
in	O
terms	O
of	O
DNS	B-Protocol
zone	O
location	O
in	O
the	O
hierarchy	O
of	O
DNS	B-Protocol
labels	O
:	O
it	O
can	O
be	O
interpreted	O
only	O
in	O
one	O
way	O
.	O
</s>
<s>
A	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
is	O
conventionally	O
written	O
as	O
a	O
list	O
of	O
domain	O
labels	O
separated	O
using	O
the	O
full	O
stop	O
“”	O
character	O
(	O
dot	O
or	O
period	O
)	O
.	O
</s>
<s>
The	O
top	O
of	O
the	O
hierarchy	O
in	O
an	O
FQDN	B-Protocol
begins	O
with	O
the	O
rightmost	O
label	O
.	O
</s>
<s>
For	O
instance	O
,	O
in	O
the	O
FQDN	B-Protocol
,	O
is	O
a	O
label	O
directly	O
under	O
the	O
root	B-Protocol
zone	I-Protocol
,	O
is	O
nested	O
under	O
,	O
and	O
finally	O
is	O
nested	O
under	O
.	O
</s>
<s>
The	O
topmost	O
layer	O
of	O
every	O
domain	O
name	O
is	O
the	O
DNS	B-Protocol
root	I-Protocol
zone	I-Protocol
,	O
which	O
is	O
expressed	O
as	O
an	O
empty	O
label	O
and	O
can	O
be	O
represented	O
in	O
an	O
FQDN	B-Protocol
with	O
a	O
trailing	O
dot	O
,	O
such	O
as	O
.	O
</s>
<s>
Trailing	O
dots	O
are	O
required	O
by	O
the	O
standard	O
format	O
for	O
DNS	B-Protocol
zone	B-Protocol
files	I-Protocol
,	O
as	O
well	O
as	O
to	O
disambiguate	O
cases	O
where	O
an	O
FQDN	B-Protocol
does	O
not	O
contain	O
any	O
other	O
label	O
separators	O
,	O
such	O
as	O
the	O
FQDNs	B-Protocol
for	O
the	O
root	B-Protocol
zone	I-Protocol
itself	O
and	O
any	O
top-level	O
domain	O
.	O
</s>
<s>
It	O
may	O
also	O
be	O
referred	O
to	O
as	O
a	O
partially-qualified	O
domain	O
name	O
,	O
or	O
PQDN	B-Protocol
.	O
</s>
<s>
Hostnames	B-Protocol
can	O
be	O
used	O
as	O
relative	O
domain	O
names	O
.	O
</s>
<s>
Dot-separated	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
names	I-Protocol
are	O
the	O
primarily	O
used	O
form	O
for	O
human-readable	O
representations	O
of	O
a	O
domain	O
name	O
.	O
</s>
<s>
Dot-separated	O
domain	O
names	O
are	O
not	O
used	O
in	O
the	O
internal	O
representation	O
of	O
labels	O
in	O
a	O
DNS	B-Protocol
message	O
but	O
are	O
used	O
to	O
reference	O
domains	O
in	O
some	O
TXT	B-Protocol
records	I-Protocol
and	O
can	O
appear	O
in	O
resolver	O
configurations	O
,	O
system	O
hosts	B-Protocol
files	I-Protocol
,	O
and	O
URLs	O
.	O
</s>
<s>
Web	O
addresses	O
typically	O
use	O
FQDNs	B-Protocol
to	O
represent	O
the	O
host	O
,	O
as	O
it	O
ensures	O
the	O
address	O
will	O
be	O
interpreted	O
identically	O
on	O
any	O
network	O
.	O
</s>
<s>
Relative	O
hostnames	B-Protocol
are	O
allowed	O
by	O
some	O
protocols	O
,	O
including	O
HTTP	B-Protocol
,	O
but	O
disallowed	O
by	O
others	O
,	O
such	O
as	O
the	O
Simple	B-Protocol
Mail	I-Protocol
Transfer	I-Protocol
Protocol	I-Protocol
(	O
SMTP	B-Protocol
)	O
.	O
</s>
