<s>
In	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
hierarchy	O
,	O
a	O
subdomain	B-Protocol
is	O
a	O
domain	O
that	O
is	O
a	O
part	O
of	O
another	O
(	O
main	O
)	O
domain	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
domain	O
offered	O
an	O
online	O
store	O
as	O
part	O
of	O
their	O
website	O
example.com,	O
it	O
might	O
use	O
the	O
subdomain	B-Protocol
shop.example.com	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
has	O
a	O
tree	B-Data_Structure
structure	I-Data_Structure
or	O
hierarchy	O
,	O
which	O
includes	O
nodes	O
on	O
the	O
tree	O
being	O
a	O
domain	O
name	O
.	O
</s>
<s>
A	O
subdomain	B-Protocol
is	O
a	O
domain	O
that	O
is	O
part	O
of	O
a	O
larger	O
domain	O
.	O
</s>
<s>
Subdomains	B-Protocol
are	O
defined	O
by	O
editing	O
the	O
DNS	B-Protocol
zone	O
file	O
pertaining	O
to	O
the	O
parent	O
domain	O
.	O
</s>
<s>
However	O
,	O
there	O
is	O
an	O
ongoing	O
debate	O
over	O
the	O
use	O
of	O
the	O
term	O
"	O
subdomain	B-Protocol
"	O
when	O
referring	O
to	O
names	O
which	O
map	O
to	O
the	O
Address	O
record	O
A	O
(	O
host	O
)	O
and	O
various	O
other	O
types	O
of	O
zone	O
records	O
which	O
may	O
map	O
to	O
any	O
public	O
IP	B-Protocol
address	I-Protocol
destination	O
and	O
any	O
type	O
of	O
server	O
.	O
</s>
<s>
Network	O
Operations	O
teams	O
insist	O
that	O
it	O
is	O
inappropriate	O
to	O
use	O
the	O
term	O
"	O
subdomain	B-Protocol
"	O
to	O
refer	O
to	O
any	O
mapping	O
other	O
than	O
that	O
provided	O
by	O
zone	O
NS	O
(	O
name	O
server	O
)	O
records	O
and	O
any	O
server-destination	O
other	O
than	O
that	O
.	O
</s>
<s>
According	O
to	O
RFC	O
1034	O
,	O
"	O
a	O
domain	O
is	O
a	O
subdomain	B-Protocol
of	O
another	O
domain	O
if	O
it	O
is	O
contained	O
within	O
that	O
domain	O
"	O
.	O
</s>
<s>
Based	O
on	O
that	O
definition	O
,	O
a	O
host	O
cannot	O
be	O
a	O
subdomain	B-Protocol
,	O
only	O
a	O
domain	O
can	O
be	O
a	O
subdomain	B-Protocol
.	O
</s>
<s>
A	O
subdomain	B-Protocol
will	O
also	O
have	O
a	O
separate	O
zone	O
file	O
with	O
a	O
SOA	O
record	O
(	O
Start	O
of	O
Authority	O
)	O
.	O
</s>
<s>
Most	O
domain	B-Protocol
registries	I-Protocol
only	O
allocate	O
a	O
two-level	O
domain	O
name	O
.	O
</s>
<s>
Hosting	O
services	O
typically	O
provide	O
DNS	B-Protocol
Servers	O
to	O
resolve	O
subdomains	B-Protocol
within	O
that	O
master	O
domain	O
.	O
</s>
<s>
The	O
en	O
is	O
a	O
subdomain	B-Protocol
of	O
wikipedia.org	O
.	O
</s>
<s>
Although	O
wikipedia.org	O
is	O
usually	O
considered	O
to	O
be	O
the	O
domain	O
name	O
,	O
wikipedia	O
is	O
actually	O
a	O
sub-domain	B-Protocol
of	O
the	O
org	O
TLD	O
(	O
top	O
level	O
domain	O
)	O
.	O
</s>
<s>
Any	O
fully	O
qualified	O
domain	O
name	O
can	O
be	O
a	O
host	O
or	O
a	O
subdomain	B-Protocol
.	O
</s>
<s>
A	O
domain	O
name	O
that	O
does	O
not	O
include	O
any	O
subdomains	B-Protocol
is	O
known	O
as	O
an	O
apex	B-Protocol
domain	I-Protocol
,	O
root	O
domain	O
,	O
or	O
bare	O
domain	O
.	O
</s>
<s>
For	O
example	O
,	O
wikipedia.org	O
is	O
the	O
apex	B-Protocol
domain	I-Protocol
of	O
Wikipedia	O
,	O
which	O
redirects	O
to	O
the	O
subdomain	B-Protocol
www.wikipedia.org	O
.	O
</s>
<s>
Subdomains	B-Protocol
are	O
often	O
used	O
by	O
internet	O
service	O
providers	O
supplying	O
web	O
services	O
.	O
</s>
<s>
They	O
allocate	O
one	O
(	O
or	O
more	O
)	O
subdomains	B-Protocol
to	O
their	O
clients	O
who	O
do	O
not	O
have	O
their	O
own	O
domain	O
name	O
.	O
</s>
<s>
This	O
allows	O
independent	O
administration	O
by	O
the	O
clients	O
over	O
their	O
subdomain	B-Protocol
.	O
</s>
<s>
Subdomains	B-Protocol
are	O
also	O
used	O
by	O
organizations	O
that	O
wish	O
to	O
assign	O
a	O
unique	O
name	O
to	O
a	O
particular	O
department	O
,	O
function	O
,	O
or	O
service	O
related	O
to	O
the	O
organization	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
university	O
might	O
assign	O
"	O
cs	O
"	O
to	O
the	O
computer	O
science	O
department	O
,	O
such	O
that	O
a	O
number	O
of	O
hosts	O
could	O
be	O
used	O
inside	O
that	O
subdomain	B-Protocol
,	O
such	O
as	O
www.cs.example.edu	O
.	O
</s>
<s>
There	O
are	O
some	O
widely	O
recognized	O
subdomains	B-Protocol
such	O
as	O
WWW	O
and	O
FTP	B-Protocol
.	O
</s>
<s>
This	O
allows	O
for	O
a	O
structure	O
where	O
the	O
domain	O
contains	O
administrative	O
directories	B-Application
and	O
files	O
including	O
the	O
FTP	B-Protocol
directories	B-Application
and	O
webpages	O
.	O
</s>
<s>
The	O
FTP	B-Protocol
subdomain	B-Protocol
could	O
contain	O
logs	O
and	O
the	O
web	O
page	O
directories	B-Application
,	O
while	O
the	O
WWW	O
subdomain	B-Protocol
contains	O
the	O
directories	B-Application
for	O
the	O
webpages	O
.	O
</s>
<s>
A	O
vanity	B-Protocol
domain	I-Protocol
is	O
a	O
subdomain	B-Protocol
of	O
an	O
ISP	O
's	O
domain	O
that	O
is	O
aliased	O
to	O
an	O
individual	O
user	O
account	O
,	O
or	O
a	O
subdomain	B-Protocol
that	O
expresses	O
the	O
individuality	O
of	O
the	O
person	O
on	O
whose	O
behalf	O
it	O
is	O
registered	O
.	O
</s>
<s>
Depending	O
on	O
application	O
,	O
a	O
record	O
inside	O
a	O
domain	O
,	O
or	O
subdomain	B-Protocol
might	O
refer	O
to	O
a	O
hostname	B-Protocol
,	O
or	O
a	O
service	O
provided	O
by	O
a	O
number	O
of	O
machines	O
in	O
a	O
cluster	O
.	O
</s>
<s>
Some	O
websites	O
use	O
different	O
subdomains	B-Protocol
to	O
point	O
to	O
different	O
server	B-Architecture
clusters	I-Architecture
.	O
</s>
<s>
For	O
example	O
,	O
www.example.com	O
points	O
to	O
Server	B-Architecture
Cluster	I-Architecture
1	O
or	O
Datacentre	O
1	O
,	O
and	O
www2.example.com	O
points	O
to	O
Server	B-Architecture
Cluster	I-Architecture
2	O
or	O
Datacentre	O
2	O
etc	O
.	O
</s>
<s>
Subdomains	B-Protocol
are	O
different	O
from	O
directories	B-Application
.	O
</s>
<s>
Directories	B-Application
are	O
physical	O
folders	O
on	O
an	O
actual	O
computer	O
,	O
while	O
subdomains	B-Protocol
are	O
a	O
part	O
of	O
the	O
URL	O
that	O
can	O
be	O
routed	O
to	O
any	O
file	O
or	O
folder	O
on	O
the	O
server	O
machine	O
.	O
</s>
