<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
hostname	B-Protocol
(	O
archaically	O
nodename	B-Protocol
)	O
is	O
a	O
label	O
that	O
is	O
assigned	O
to	O
a	O
device	O
connected	O
to	O
a	O
computer	B-Architecture
network	I-Architecture
and	O
that	O
is	O
used	O
to	O
identify	O
the	O
device	O
in	O
various	O
forms	O
of	O
electronic	O
communication	O
,	O
such	O
as	O
the	O
World	O
Wide	O
Web	O
.	O
</s>
<s>
Hostnames	B-Protocol
may	O
be	O
simple	O
names	O
consisting	O
of	O
a	O
single	O
word	O
or	O
phrase	O
,	O
or	O
they	O
may	O
be	O
structured	O
.	O
</s>
<s>
Each	O
hostname	B-Protocol
usually	O
has	O
at	O
least	O
one	O
numeric	O
network	B-Protocol
address	I-Protocol
associated	O
with	O
it	O
for	O
routing	O
packets	O
for	O
performance	O
and	O
other	O
reasons	O
.	O
</s>
<s>
Internet	O
hostnames	B-Protocol
may	O
have	O
appended	O
the	O
name	O
of	O
a	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
domain	O
,	O
separated	O
from	O
the	O
host-specific	O
label	O
by	O
a	O
period	O
(	O
"	O
dot	O
"	O
)	O
.	O
</s>
<s>
In	O
the	O
latter	O
form	O
,	O
a	O
hostname	B-Protocol
is	O
also	O
called	O
a	O
domain	O
name	O
.	O
</s>
<s>
If	O
the	O
domain	O
name	O
is	O
completely	O
specified	O
,	O
including	O
a	O
top-level	O
domain	O
of	O
the	O
Internet	O
,	O
then	O
the	O
hostname	B-Protocol
is	O
said	O
to	O
be	O
a	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
(	O
FQDN	B-Protocol
)	O
.	O
</s>
<s>
Hostnames	B-Protocol
that	O
include	O
DNS	B-Protocol
domains	O
are	O
often	O
stored	O
in	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
together	O
with	O
the	O
IP	B-Protocol
addresses	I-Protocol
of	O
the	O
host	O
they	O
represent	O
for	O
the	O
purpose	O
of	O
mapping	O
the	O
hostname	B-Protocol
to	O
an	O
address	O
,	O
or	O
the	O
reverse	O
process	O
.	O
</s>
<s>
In	O
the	O
Internet	O
,	O
a	O
hostname	B-Protocol
is	O
a	O
domain	O
name	O
assigned	O
to	O
a	O
host	O
computer	O
.	O
</s>
<s>
For	O
example	O
,	O
en.wikipedia.org	O
consists	O
of	O
a	O
local	O
hostname	B-Protocol
(	O
en	O
)	O
and	O
the	O
domain	O
name	O
wikipedia.org	O
.	O
</s>
<s>
This	O
kind	O
of	O
hostname	B-Protocol
is	O
translated	O
into	O
an	O
IP	B-Protocol
address	I-Protocol
via	O
the	O
local	O
hosts	B-Protocol
file	I-Protocol
,	O
or	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
resolver	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
a	O
single	O
host	O
computer	O
to	O
have	O
several	O
hostnames	B-Protocol
;	O
but	O
generally	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
the	O
host	O
prefers	O
to	O
have	O
one	O
hostname	B-Protocol
that	O
the	O
host	O
uses	O
for	O
itself	O
.	O
</s>
<s>
Any	O
domain	O
name	O
can	O
also	O
be	O
a	O
hostname	B-Protocol
,	O
as	O
long	O
as	O
the	O
restrictions	O
mentioned	O
below	O
are	O
followed	O
.	O
</s>
<s>
So	O
,	O
for	O
example	O
,	O
both	O
en.wikipedia.org	O
and	O
wikipedia.org	O
are	O
hostnames	B-Protocol
because	O
they	O
both	O
have	O
IP	B-Protocol
addresses	I-Protocol
assigned	O
to	O
them	O
.	O
</s>
<s>
A	O
hostname	B-Protocol
may	O
be	O
a	O
domain	O
name	O
,	O
if	O
it	O
is	O
properly	O
organized	O
into	O
the	O
domain	B-Protocol
name	I-Protocol
system	I-Protocol
.	O
</s>
<s>
A	O
domain	O
name	O
may	O
be	O
a	O
hostname	B-Protocol
if	O
it	O
has	O
been	O
assigned	O
to	O
an	O
Internet	O
host	O
and	O
associated	O
with	O
the	O
host	O
's	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Hostnames	B-Protocol
are	O
composed	O
of	O
a	O
sequence	O
of	O
labels	O
concatenated	O
with	O
dots	O
.	O
</s>
<s>
For	O
example	O
,	O
"	O
en.wikipedia.org	O
 "	O
is	O
a	O
hostname	B-Protocol
.	O
</s>
<s>
The	O
entire	O
hostname	B-Protocol
,	O
including	O
the	O
delimiting	O
dots	O
,	O
has	O
a	O
maximum	O
of	O
253	O
ASCII	B-Protocol
characters	I-Protocol
.	O
</s>
<s>
The	O
Internet	O
standards	O
(	O
Request	O
for	O
Comments	O
)	O
for	O
protocols	O
specify	O
that	O
labels	O
may	O
contain	O
only	O
the	O
ASCII	B-Protocol
letters	I-Protocol
a	O
through	O
z	O
(	O
in	O
a	O
case-insensitive	O
manner	O
)	O
,	O
the	O
digits	O
0	O
through	O
9	O
,	O
and	O
the	O
hyphen-minus	O
character	O
('-'	O
)	O
.	O
</s>
<s>
The	O
original	O
specification	O
of	O
hostnames	B-Protocol
required	O
that	O
labels	O
start	O
with	O
an	O
alpha	O
character	O
,	O
and	O
not	O
end	O
with	O
a	O
hyphen	O
.	O
</s>
<s>
However	O
,	O
a	O
subsequent	O
specification	O
permitted	O
hostname	B-Protocol
labels	O
to	O
start	O
with	O
digits	O
.	O
</s>
<s>
Internationalized	B-Language
domain	I-Language
names	I-Language
are	O
stored	O
in	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
as	O
ASCII	B-Protocol
strings	O
using	O
Punycode	B-General_Concept
transcription	O
.	O
</s>
<s>
While	O
a	O
hostname	B-Protocol
may	O
not	O
contain	O
other	O
characters	O
,	O
such	O
as	O
the	O
underscore	O
character	O
( _	O
)	O
,	O
other	O
DNS	B-Protocol
names	I-Protocol
may	O
contain	O
the	O
underscore	O
.	O
</s>
<s>
Systems	O
such	O
as	O
DomainKeys	B-Protocol
and	O
service	B-Protocol
records	I-Protocol
use	O
the	O
underscore	O
as	O
a	O
means	O
to	O
assure	O
that	O
their	O
special	O
character	O
is	O
not	O
confused	O
with	O
hostnames	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
_http._sctp.www.example.com	O
specifies	O
a	O
service	O
pointer	O
for	O
an	O
SCTP-capable	O
webserver	O
host	O
(	O
www	O
)	O
in	O
the	O
domain	O
example.com	O
.	O
</s>
<s>
Notwithstanding	O
the	O
standard	O
,	O
Chrome	O
,	O
Firefox	B-Application
,	O
Internet	B-Application
Explorer	I-Application
,	O
Edge	B-Application
and	O
Safari	B-Application
allow	O
underscores	O
in	O
hostnames	B-Protocol
,	O
although	O
cookies	O
in	O
IE	O
do	O
not	O
work	O
correctly	O
if	O
any	O
part	O
of	O
the	O
hostname	B-Protocol
contains	O
an	O
underscore	O
character	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
valid	O
to	O
attempt	O
to	O
resolve	O
a	O
hostname	B-Protocol
that	O
consists	O
of	O
an	O
underscore	O
.	O
</s>
<s>
This	O
is	O
used	O
by	O
RFC	O
7816	O
to	O
reduce	O
the	O
amount	O
of	O
information	O
that	O
is	O
made	O
available	O
to	O
intermediate	O
DNS	B-Protocol
servers	O
during	O
an	O
iterative	O
query	O
.	O
</s>
<s>
The	O
hostname	B-Protocol
en.wikipedia.org	O
is	O
composed	O
of	O
the	O
DNS	B-Protocol
labels	O
en	O
(	O
hostname	B-Protocol
or	O
leaf	O
domain	O
)	O
,	O
wikipedia	O
(	O
second-level	O
domain	O
)	O
and	O
org	O
(	O
top-level	O
domain	O
)	O
.	O
</s>
<s>
Labels	O
such	O
as	O
2600	O
and	O
3abc	O
may	O
be	O
used	O
in	O
hostnames	B-Protocol
,	O
but	O
-hi-	O
,	O
_hi_	O
and	O
*	O
hi*	O
are	O
invalid	O
.	O
</s>
<s>
A	O
hostname	B-Protocol
is	O
considered	O
to	O
be	O
a	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
(	O
FQDN	B-Protocol
)	O
when	O
all	O
labels	O
up	O
to	O
and	O
including	O
the	O
top-level	O
domain	O
name	O
(	O
TLD	O
)	O
are	O
specified	O
.	O
</s>
<s>
The	O
hostname	B-Protocol
en.wikipedia.org	O
terminates	O
with	O
the	O
top-level	O
domain	O
org	O
and	O
is	O
thus	O
fully	O
qualified	O
.	O
</s>
<s>
Depending	O
on	O
the	O
operating	B-General_Concept
system	I-General_Concept
DNS	B-Protocol
software	O
implementation	O
,	O
an	O
unqualified	O
hostname	B-Protocol
may	O
be	O
automatically	O
combined	O
with	O
a	O
default	O
domain	O
name	O
configured	O
into	O
the	O
system	O
,	O
in	O
order	O
to	O
complete	O
the	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
.	O
</s>
<s>
General	O
guidelines	O
on	O
choosing	O
a	O
good	O
hostname	B-Protocol
are	O
outlined	O
in	O
RFC	O
1178	O
.	O
</s>
<s>
saturn	O
and	O
jupiter	O
may	O
be	O
the	O
hostnames	B-Protocol
of	O
two	O
devices	O
connected	O
to	O
a	O
network	O
named	O
PC	O
.	O
</s>
<s>
Within	O
PC	O
,	O
the	O
devices	O
are	O
addressed	O
by	O
their	O
hostnames	B-Protocol
.	O
</s>
<s>
If	O
PC	O
is	O
registered	O
as	O
a	O
second-level	O
domain	O
name	O
in	O
the	O
Internet	O
,	O
e.g.	O
,	O
as	O
,	O
the	O
hosts	B-Protocol
may	O
be	O
addressed	O
by	O
the	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
names	I-Protocol
and	O
.	O
</s>
