Reverse DNS Zone Generator

Forward DNS turns names into addresses. Reverse DNS turns an address back into a name using PTR records, and those live in special zones under in-addr.arpa for IPv4 and ip6.arpa for IPv6. Setting them up by hand is fiddly because the labels run backwards. The page works out the zone name, the name of each address and a set of sample records.

How the names are formed

For IPv4, write the address in reverse order and append in-addr.arpa: 192.0.2.10 becomes 10.2.0.192.in-addr.arpa. For IPv6, write all 32 hexadecimal digits with no omitted zeros, reverse their order, put a dot between every digit and append ip6.arpa. The address 2001:db8::567:89ab therefore starts b.a.9.8.7.6.5.0 and ends 8.b.d.0.1.0.0.2.ip6.arpa, with sixteen zero labels in between. Reversing is needed because DNS delegates from the right, so the most significant part of the address has to be the rightmost label.

Where a zone can be delegated

Delegation happens only at label boundaries. In IPv4 that means whole octets: a /24 uses a zone of three labels, for example 2.0.192.in-addr.arpa, a /16 uses two, and a /8 uses one. In IPv6 it means whole hexadecimal digits, which is every 4 bits. A prefix that does not end on a boundary does not have a zone of its own. For blocks smaller than a /24, such as 203.0.113.128/25, RFC 2317 describes classless delegation: the parent zone holds CNAME records that point into a child zone whose name carries the range, for instance 128-255.113.0.203.in-addr.arpa. The page explains which case applies and prints the right zone.

What you enter and what you get

Type a single address or a CIDR block, a host name pattern and a domain, or press a sample chip. The result lists the reverse zone to create or ask for, the reverse name of the first address, and a block of sample PTR records in zone-file syntax, ready to copy into a zone. For a /24 the sample runs through the block with names built from the address, and you replace the pattern with your real host names.

A worked example

Choose 192.0.2.0/24. The page reports the zone 2.0.192.in-addr.arpa and notes that the prefix ends on an octet boundary, so a plain zone covers it. The first address is named 0.2.0.192.in-addr.arpa. The sample records begin 0.2.0.192.in-addr.arpa. IN PTR host-192-0-2-0.example.com. and continue with 1.2.0.192.in-addr.arpa. IN PTR host-192-0-2-1.example.com. and so on through the block. 192.0.2.0/24 is a documentation range, so these names are for practice only.

Limits

The page writes names and sample records; it does not publish them or verify that your provider has delegated the zone. After you have records in place, check them with the Reverse DNS Lookup, which also confirms that each name resolves back to the same address. For a full zone file around the records, use the BIND Zone File Generator.

Related tools

• Reverse DNS Lookup queries real PTR records and checks forward confirmation.
• BIND Zone File Generator builds a whole zone file.
• IPv6 Expander gives the 32 digits that ip6.arpa names are made from.
• Subnet Calculator shows the range a delegated block covers.