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Reverse Zones

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Please note that Reverse DNS is usually performed by your ISP because they control the DNS for your IP numbers. They would need to delegate a reverse zone to WorldwideDNS in order for reverse zones you set up on our system to work. It is generally much easier for your ISP to perform the reverse DNS and most will do it at no additional charge.

The Internet uses the special "" domain to support reverse (IP address to domain name) look-ups.

An IP address is represented in this domain by reversing the order of the segments in the IP address and adding "". IP would be represented by the domain name "".

A PTR-record is used to point this back to the name of an A-record representing the IP address.

The "" domain is delegated like other domains (see NS-records), so if you are assigned the class C network 1.2.3.X, you can also be delegated DNS authority for the the "" domain name. Your DNS servers should in this case have a zone called "" containing PTR-records for all active IP addresses in the class C network ( to

It is also possible to delegate "" authority for less than one class C network. This can be achieved in different ways, but typically follows the style described in RFC2317. Please note - if you only have a few IP addresses, your ISP will probably maintain the reverse records for you and not delegate them.

For example if you are assigned network ( to subnet mask, the owner of the class C 1.2.3.X would have these DNS entries:

NS 24/ = your-dns-server-name1
NS 24/ = your-dns-server-name2
NS 24/ = your-dns-server-name3
CNAME = 25.24/
CNAME = 26.24/
CNAME = 27.24/
CNAME = 28.24/
CNAME = 29.24/
CNAME = 30.24/

Your DNS server would have a zone named "24/" with the following records:

@ NS your-dns-server-name1
@ NS your-dns-server-name2
@ NS your-dns-server-name3
25 PTR name1.your-domain-name
26 PTR name2.your-domain-name
27 PTR name3.your-domain-name
28 PTR name4.your-domain-name
29 PTR name5.your-domain-name
30 PTR name6.your-domain-name

A reverse lookup for IP (PTR-record for ""), would first return an alias (CNAME-record) for "27.24/" from the class C owner's DNS server, which is then translated to "name3.your-domain-name" by your DNS server.

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