Reverse DNS is IP address to domain name mapping - the opposite of forward (normal) DNS which maps domain names to IP addresses.
Reverse DNS is maintained in a separate set of data from forward DNS.
For example, forward DNS for "abc.com" pointing to IP address "1.2.3.4", does not necessarily mean that reverse DNS for IP "1.2.3.4" also points to "abc.com".
Reverse DNS is mostly used by humans for such things as tracking where a web-site visitor came from, or where an e-mail message originated etc.
Reverse DNS is typically not as critical in as forward DNS - visitors will still reach your web-site just fine without any reverse DNS for your web-server IP or the visitor's IP.
However there is one important exception: Many e-mail servers on the Internet (including AOL's) are configured to reject incoming e-mails from any IP address which does not have reverse DNS.
So if you run your own e-mail server, reverse DNS must exist for the IP address that outgoing e-mail is sent from.
It does not matter what the reverse DNS record for your IP address points to as long as it is there. If you host multiple domains on one e-mail server, just setup reverse DNS to point to whichever domain name you consider primary.
(e-mail servers checking for reverse DNS know that it is normal to host many domains on a single IP address and it would be impossible to list all those domains in reverse DNS for the IP).
A special PTR-record type is used to store reverse DNS entries. The name of a PTR-record is the IP address with the segments reversed + ".in-addr.arpa". For example the reverse DNS entry for IP 1.2.3.4 would be stored as a PTR-record for "4.3.2.1.in-addr.arpa".
In Simple DNS Plus, a zone for reverse DNS records is created using the New Zone function in the DNS Records window.
Simple DNS Plus provides a Reverse Zone Wizard which makes it easy to maintain reverse zones and records (without dealing with "in-addr.arpa", reversing IP addresses etc.)
Reverse records can also be created automatically by checking "Update reverse zone" when entering A-Records through the Record Properties dialog.
Reverse DNS is also different from forward DNS in who points (delegates) the zone to your DNS server.
With forward DNS, you point the zone to your DNS server by registering that domain name with a registrar.
With reverse DNS, your Internet connection provider (ISP) must point the zone ("....in-addr.arpa") to your DNS server.
Without this delegation from your ISP, your reverse zone will not work.
If you are assigned the class C network 1.2.3.X, your ISP can delegate DNS authority for the "3.2.1.in-addr.arpa" domain name to your DNS server.
Your DNS servers should in this case have a zone called "3.2.1.in-addr.arpa" containing PTR-records for all active IP addresses in the class C network (1.2.3.0 - 1.2.3.255).
It is also possible to delegate "in-addr.arpa" authority for less than one class C network (256 IP addresses).
This can be achieved in different ways, but typically follows the style described in RFC2317.
(Please note: Many ISPs will not do this sub-delegation if you only have one or a few IP addresses. In this case your ISP has probably already setup some default reverse DNS for your IP addresses)
For example if you are assigned network 1.2.3.24/29 (1.2.3.25 to 1.2.3.30 subnet mask 255.255.255.248), the owner of the class C 1.2.3.X (your ISP) would have these DNS entries on his DNS server:
A reverse lookup for IP 1.2.3.27 (PTR-record for "27.3.2.1.in-addr.arpa"), would first return an alias (CNAME-record) for "27.24/29.3.2.1.in-addr.arpa" from the class C owner's DNS server, which is then translated to "name3.your-domain-name" by your DNS server.
