<s>
A	O
DNS	B-Protocol
zone	I-Protocol
is	O
a	O
specific	O
portion	O
of	O
the	O
DNS	B-Protocol
namespace	O
in	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
,	O
which	O
a	O
specific	O
organization	O
or	O
administrator	O
manages	O
.	O
</s>
<s>
A	O
DNS	B-Protocol
zone	I-Protocol
is	O
an	O
administrative	O
space	O
allowing	O
more	O
granular	O
control	O
of	O
the	O
DNS	B-Protocol
components	O
,	O
such	O
as	O
authoritative	O
nameserver	B-Protocol
.	O
</s>
<s>
The	O
DNS	B-Protocol
is	O
broken	O
up	O
into	O
different	O
zones	O
,	O
distinctly	O
managed	O
areas	O
in	O
the	O
DNS	B-Protocol
namespace	O
.	O
</s>
<s>
DNS	B-Protocol
zones	I-Protocol
are	O
not	O
necessarily	O
physically	O
separated	O
from	O
one	O
another	O
;	O
however	O
,	O
a	O
DNS	B-Protocol
zone	I-Protocol
can	O
contain	O
multiple	O
subdomains	B-Protocol
,	O
and	O
multiple	O
zones	O
can	O
exist	O
on	O
the	O
same	O
server	O
.	O
</s>
<s>
The	O
domain	B-Protocol
namespace	I-Protocol
of	O
the	O
Internet	O
is	O
organized	O
into	O
a	O
hierarchical	O
layout	O
of	O
subdomains	B-Protocol
below	O
the	O
DNS	B-Protocol
root	I-Protocol
domain	O
.	O
</s>
<s>
A	O
zone	O
starts	O
at	O
a	O
domain	O
and	O
extends	O
downward	O
in	O
the	O
tree	O
to	O
the	O
leaf	O
nodes	O
or	O
to	O
the	O
top-level	O
of	O
subdomains	B-Protocol
where	O
other	O
zones	O
start	O
.	O
</s>
<s>
A	O
DNS	B-Protocol
zone	I-Protocol
is	O
implemented	O
in	O
the	O
configuration	O
system	O
of	O
a	O
domain	B-Protocol
name	I-Protocol
server	I-Protocol
.	O
</s>
<s>
Historically	O
,	O
it	O
is	O
defined	O
in	O
the	O
zone	B-Protocol
file	I-Protocol
,	O
an	O
operating	O
system	O
text	B-General_Concept
file	I-General_Concept
that	O
starts	O
with	O
the	O
special	O
DNS	B-Protocol
record	O
type	O
Start	O
of	O
Authority	O
(	O
SOA	O
)	O
and	O
contains	O
all	O
records	O
for	O
the	O
resources	O
described	O
within	O
the	O
zone	O
.	O
</s>
<s>
This	O
format	O
was	O
originally	O
used	O
by	O
the	O
Berkeley	B-Protocol
Internet	I-Protocol
Name	I-Protocol
Domain	I-Protocol
Server	I-Protocol
(	O
BIND	B-Protocol
)	O
software	O
package	O
and	O
is	O
defined	O
in	O
RFC	O
1034	O
and	O
RFC	O
1035	O
.	O
</s>
<s>
Most	O
top-level	O
domain	B-Protocol
name	I-Protocol
registry	I-Protocol
operators	O
offer	O
their	O
namespaces	O
to	O
the	O
public	O
or	O
entities	O
with	O
the	O
mandated	O
geographic	O
or	O
otherwise	O
scoped	O
purpose	O
for	O
registering	O
second-level	O
domains	O
.	O
</s>
<s>
Each	O
registration	O
or	O
allocation	O
of	O
subdomain	B-Protocol
space	O
obligates	O
the	O
registrant	O
to	O
maintain	O
an	O
administrative	O
and	O
technical	O
infrastructure	O
to	O
manage	O
the	O
responsibility	O
for	O
its	O
zone	O
,	O
including	O
sub-delegation	O
to	O
lower-level	O
domains	O
.	O
</s>
<s>
An	O
area	O
of	O
one	O
or	O
more	O
subdomains	B-Protocol
that	O
have	O
been	O
delegated	O
for	O
management	O
is	O
called	O
a	O
DNS	B-Protocol
zone	I-Protocol
.	O
</s>
<s>
As	O
each	O
domain	O
is	O
further	O
divided	O
into	O
sub-domains	B-Protocol
,	O
each	O
becoming	O
a	O
DNS	B-Protocol
zone	I-Protocol
with	O
its	O
own	O
set	O
of	O
administrators	O
and	O
DNS	B-Protocol
servers	I-Protocol
,	O
the	O
tree	O
grows	O
with	O
the	O
largest	O
number	O
of	O
leaf	O
nodes	O
at	O
the	O
bottom	O
.	O
</s>
<s>
At	O
this	O
lowest	O
level	O
,	O
in	O
the	O
end-nodes	O
or	O
leaves	O
of	O
the	O
tree	O
,	O
the	O
term	O
DNS	B-Protocol
zone	I-Protocol
becomes	O
essentially	O
synonymous	O
with	O
the	O
term	O
"	O
domain	O
"	O
,	O
both	O
in	O
terms	O
of	O
use	O
and	O
administration	O
.	O
</s>
<s>
The	O
term	O
domain	O
is	O
used	O
in	O
the	O
business	O
functions	O
of	O
the	O
entity	O
assigned	O
to	O
it	O
,	O
and	O
the	O
term	O
zone	O
is	O
usually	O
used	O
for	O
the	O
configuration	O
of	O
DNS	B-Protocol
services	O
.	O
</s>
<s>
DNS	B-Protocol
zones	I-Protocol
contain	O
the	O
records	O
for	O
mapping	O
domain	O
names	O
to	O
IP	B-Protocol
addresses	I-Protocol
or	O
other	O
information	O
.	O
</s>
<s>
The	O
resolution	O
of	O
a	O
domain	O
name	O
to	O
its	O
assigned	O
information	O
is	O
also	O
referred	O
to	O
as	O
forward	O
resolution	O
,	O
and	O
the	O
DNS	B-Protocol
zones	I-Protocol
associated	O
with	O
such	O
processes	O
are	O
often	O
referred	O
to	O
as	O
forward	O
zones	O
.	O
</s>
<s>
The	O
term	O
arose	O
as	O
the	O
opposite	O
of	O
reverse	O
zones	O
,	O
which	O
are	O
used	O
for	O
the	O
reverse	O
process	O
:	O
finding	O
the	O
DNS	B-Protocol
name	I-Protocol
associated	O
with	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Such	O
reverse	O
zones	O
are	O
maintained	O
in	O
the	O
Internet	B-Protocol
Address	I-Protocol
and	O
Routing	O
Parameter	O
Area	O
(	O
domain	O
arpa	O
)	O
.	O
</s>
<s>
Another	O
common	O
use	O
of	O
the	O
term	O
forward	O
zone	O
refers	O
to	O
a	O
specific	O
configuration	O
of	O
DNS	B-Protocol
name	B-Protocol
servers	I-Protocol
,	O
particularly	O
caching	O
name	B-Protocol
servers	I-Protocol
,	O
in	O
which	O
resolution	O
of	O
a	O
domain	O
name	O
is	O
forwarded	O
to	O
another	O
name	B-Protocol
server	I-Protocol
that	O
is	O
authoritative	O
for	O
the	O
domain	O
in	O
question	O
,	O
rather	O
than	O
being	O
answered	O
from	O
the	O
established	O
cache	O
memory	O
.	O
</s>
<s>
The	O
top-level	O
domain	O
arpa	O
serves	O
as	O
a	O
delegation	O
zone	O
for	O
various	O
technical	O
infrastructure	O
aspects	O
of	O
DNS	B-Protocol
and	O
the	O
Internet	O
,	O
and	O
does	O
not	O
implement	O
the	O
registration	O
and	O
delegation	O
system	O
of	O
the	O
country	O
and	O
generic	O
domains	O
.	O
</s>
<s>
Intended	O
as	O
a	O
transitional	O
aid	O
to	O
the	O
DNS	B-Protocol
,	O
deleting	O
the	O
domain	O
arpa	O
was	O
later	O
found	O
to	O
be	O
impractical	O
.	O
</s>
<s>
It	O
contains	O
sub-zones	O
used	O
for	O
reverse	O
resolution	O
of	O
IP	B-Protocol
addresses	I-Protocol
to	O
host	O
names	O
(	O
IPv4	B-Protocol
:	O
in-addr.arpa	O
,	O
IPv6	B-Protocol
:	O
ip6.arpa	O
)	O
,	O
telephone	O
number	O
mapping	O
(	O
ENUM	B-Language
,	O
e164.arpa	O
)	O
,	O
and	O
uniform	O
resource	O
identifier	O
resolution	O
(	O
uri.arpa	O
,	O
urn.arpa	O
)	O
.	O
</s>
<s>
Although	O
the	O
administrative	O
structure	O
of	O
this	O
domain	O
and	O
its	O
sub-domains	B-Protocol
is	O
different	O
,	O
the	O
technical	O
delegation	O
into	O
zones	O
of	O
responsibility	O
is	O
similar	O
and	O
the	O
DNS	B-Protocol
tools	O
and	O
servers	O
used	O
are	O
identical	O
to	O
any	O
other	O
zone	O
.	O
</s>
<s>
For	O
example	O
,	O
8.8.2.5.5.2.2.0.0.8.1.e164.arpa.	O
,	O
which	O
might	O
represent	O
an	O
E.164	O
telephone	O
number	O
in	O
the	O
ENUM	B-Language
system	O
,	O
might	O
be	O
sub-delegated	O
at	O
suitable	O
boundaries	O
of	O
the	O
name	O
.	O
</s>
<s>
An	O
example	O
of	O
an	O
IP	B-Protocol
addresses	I-Protocol
in	O
the	O
reverse	O
DNS	B-Protocol
zone	I-Protocol
is	O
166.188.77.208.in-addr.arpa	O
,	O
which	O
represents	O
the	O
address	O
208.77.188.166	O
and	O
resolves	O
to	O
the	O
domain	O
name	O
www.example.com	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
IP	B-Protocol
addresses	I-Protocol
,	O
the	O
reverse	O
zones	O
are	O
delegated	O
to	O
the	O
Internet	O
service	O
provider	O
(	O
ISP	O
)	O
to	O
which	O
the	O
IP	B-Protocol
address	I-Protocol
block	O
is	O
assigned	O
.	O
</s>
<s>
When	O
an	O
ISP	O
allocates	O
a	O
range	O
to	O
a	O
customer	O
,	O
it	O
usually	O
also	O
delegates	O
the	O
management	O
of	O
that	O
space	O
to	O
the	O
customer	O
by	O
insertion	O
of	O
name	B-Protocol
server	I-Protocol
resource	O
records	O
pointing	O
to	O
the	O
customer	O
's	O
DNS	B-Protocol
facilities	O
into	O
their	O
zone	O
,	O
or	O
provides	O
other	O
management	O
tools	O
.	O
</s>
<s>
Allocations	O
of	O
single	O
IP	B-Protocol
addresses	I-Protocol
for	O
networks	O
connected	O
through	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
typically	O
do	O
not	O
provide	O
such	O
facilities	O
.	O
</s>
<s>
As	O
an	O
example	O
of	O
the	O
DNS	B-Protocol
resolving	O
process	O
,	O
consider	O
the	O
role	O
of	O
a	O
recursive	O
DNS	B-Protocol
resolver	O
attempting	O
to	O
look	O
up	O
the	O
address	O
"	O
en.wikipedia.org	O
.	O
"	O
.	O
</s>
<s>
It	O
begins	O
with	O
a	O
list	O
of	O
addresses	O
for	O
the	O
most	O
authoritative	O
name	B-Protocol
servers	I-Protocol
it	O
knows	O
about	O
–	O
the	O
root	B-Application
zone	I-Application
name	I-Application
servers	I-Application
(	O
indicated	O
by	O
the	O
full	O
stop	O
or	O
period	O
)	O
,	O
which	O
contains	O
name	B-Protocol
server	I-Protocol
information	O
for	O
all	O
top-level	O
domains	O
(	O
TLDs	O
)	O
of	O
the	O
Internet	O
.	O
</s>
<s>
When	O
querying	O
one	O
of	O
the	O
root	B-Application
name	I-Application
servers	I-Application
,	O
it	O
is	O
possible	O
that	O
the	O
root	B-Protocol
zone	I-Protocol
will	O
not	O
directly	O
contain	O
a	O
record	O
for	O
"	O
en.wikipedia.org	O
.	O
</s>
<s>
"	O
,	O
in	O
which	O
case	O
it	O
will	O
provide	O
a	O
referral	O
to	O
the	O
authoritative	O
name	B-Protocol
servers	I-Protocol
for	O
the	O
"	O
org.	O
"	O
</s>
<s>
The	O
resolver	O
is	O
issued	O
a	O
referral	O
to	O
the	O
authoritative	O
name	B-Protocol
servers	I-Protocol
for	O
the	O
"	O
org.	O
"	O
</s>
<s>
name	B-Protocol
servers	I-Protocol
,	O
the	O
resolver	O
may	O
be	O
issued	O
with	O
another	O
referral	O
to	O
the	O
"	O
wikipedia.org	O
.	O
"	O
</s>
<s>
is	O
a	O
CNAME	B-Protocol
to	O
"	O
text.wikimedia.org	O
.	O
"	O
</s>
<s>
(	O
which	O
is	O
in	O
turn	O
a	O
CNAME	B-Protocol
to	O
"	O
text.esams.wikimedia.org	O
.	O
</s>
<s>
name	B-Protocol
servers	I-Protocol
also	O
happen	O
to	O
contain	O
authoritative	O
data	O
for	O
the	O
"	O
wikimedia.org	O
.	O
"	O
</s>
<s>
zone	O
,	O
the	O
resolution	O
of	O
this	O
particular	O
query	O
occurs	O
entirely	O
within	O
the	O
queried	O
name	B-Protocol
server	I-Protocol
,	O
and	O
the	O
resolver	O
will	O
receive	O
the	O
address	O
record	O
it	O
requires	O
with	O
no	O
further	O
referrals	O
.	O
</s>
<s>
If	O
the	O
last	O
name	B-Protocol
server	I-Protocol
queried	O
did	O
not	O
contain	O
authoritative	O
data	O
for	O
the	O
target	O
of	O
the	O
CNAME	B-Protocol
,	O
it	O
would	O
have	O
issued	O
the	O
resolver	O
with	O
yet	O
another	O
referral	O
,	O
this	O
time	O
to	O
the	O
zone	O
"	O
text.wikimedia.org	O
.	O
"	O
.	O
</s>
<s>
However	O
,	O
since	O
the	O
resolver	O
had	O
previously	O
determined	O
the	O
authoritative	O
name	B-Protocol
servers	I-Protocol
for	O
the	O
zone	O
"	O
org	O
.	O
</s>
<s>
"	O
,	O
thus	O
avoiding	O
another	O
query	O
to	O
the	O
root	B-Application
name	I-Application
servers	I-Application
.	O
</s>
<s>
on	O
the	O
root	B-Application
name	I-Application
servers	I-Application
always	O
results	O
in	O
referrals	O
,	O
but	O
if	O
an	O
alternative	B-Protocol
DNS	I-Protocol
root	I-Protocol
is	O
used	O
which	O
is	O
set	O
up	O
to	O
contain	O
a	O
record	O
for	O
"	O
en.wikipedia.org	O
.	O
</s>
