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The last part of the IPv4 Address (17) is the host address and hosts, from our previous reading, are typically defined inside a zone file so we will ignore it and only use the Class C address base.
Finally, we construct a zone file to describe all the hosts (nodes) in the Reverse Mapped zone using PTR Records. We must use qualified names ending with a dot (in fact they are Fully Qualified Domain Names - FQDNs) with the PTR target (left-hand) name in reverse mapped zone files because if we did not our $ORIGIN directive would lead to some strange results. If a reverse-map file is not included in our DNS configuration then, as normal, the query will pass to the DNS hierarchy. Classless Reverse Map Delegation is defined by RFC 2317 which has Best Current Practice status and should be regarded as a definitive reference.
The technique defined in RFC 2317 provides for such delegation to take place using CNAME Resource Records (rather than the more normal PTR Resource Records) in an expanded IN-ADDR.ARPA name space. The IPv4 Reverse Map Generator can be used to generate all the necessary RRs or even just let you experiment with various delegation scenarios. Note: RFC 2317 uses the CNAME RR to implement the solution as shown in the following zone file fragments.
Now you have to change your reverse map zone names in the named.conf file to reflect the above change. The above configuration, prior to full delegation from the parent or is test mode, cannot use the normal dig command.
Instead it will continue to show the result prior to the configuration change because the dig is following the official .arpa delegation route. Unlike IPv4, where reverse mapping is frequently not delegated to the end user, IPv6 allows and encourages delegated reverse mapping. Use the IPv6 Calculator to expand IPv6 addresses and automatically generate the reverse zone name. IPv6 reverse zone names (and skeleton reverse zone files) may be generated using the IPv6 address tool. The DNAME RR may have a limited role to play in reducing files sizes in IPv6 reverse name mapping. Note: Any address not defined in either of the above zone files will, as normal, generate an NXDOMAIN (name error) response. In the case of an IPv6 end-user delegation, the normal expectation of IPv6 is that any address that is forward mapped using an AAAA (colloquially a Quad A) RR is also reverse mapped using a PTR RR. In many cases IPv6 addresses are configured using SLAAC (StateLess Address Auto Configuration) or DHCPv6 and therefore may not be known other than by manual inspection.
Note: While it is possible to use a DNS wildcard to at least provide some generic answer to the reverse-mapping problem the equivalent wildcard forward-mapping possibility does not exist. This tool uses local javascript to perform some simple manipulation of IPv4 and IPv6 prefix (or slash) notation addresses, reverse addresses, skeleton reverse map zone file generation, and a reverse map delegation zone file (as described here). The IPv4 Reverse Zone Generator can also be used to generate a delegated reverse map zone file and the corresponding CNAME or DNAME RRs for the delegater's reverse file (the reverse file at the ISP or whatever organization is delegating the reverse map). Validation or other errors will appear in Reverse Zone or IPv4 Netmask: in neither case for very good reasons. Reverse zone files can get very big, very quickly, however, to prevent unintentional errors the calculator always prompts when more than 4096 entries will appear in the reverse zone.
To create a zone file, simply copy the contents of Reverse File: to the clipboard and paste into any suitable text editor, make any required changes (you may want to review the $TTL value and the SOA serial number) and save to an appropriately named file. Where a trailing dot is essential for an FQDN (Domain Label:)the calculator will silently add it, if required, to ensure a viable zone is created. Validation or other errors will appear in Reverse Zone or IPv6 Netmask: in neither case for very good reasons.
Reverse zone files can get very big, very quickly, however, to prevent unintentional errors the calculator always prompts when more than 4096 entries will appear in the reverse zone and will flat out refuse to generate zone files with more that 65536 entries in order to save your PC, your browser and your life - in that order of importance.
This test will see if a reverse DNS entry exists for an IP address, and will also show you how the entry is found (the route of DNS servers that is taken), and who to contact to get a reverse DSN entry if none is found.

The advanced version features the server field allows you to test the reverse DNS on a new DNS server.
We write a normal domain name LEFT to RIGHT but the hierarchical structure is RIGHT to LEFT. In this case the most important part (the highest node in the address hierarchy) is on the LEFT (192) not the RIGHT.
In the case where local IP addreses are globally routable (increasingly rare) an ISP or service provider may be responsible for maintaining the reverse map in which case such reverse-map queries must access the DNS hierarchy. Classless routing allows allocation of sub-nets on non-octet boundaries, that is, less that 256 addresses from a Class C address may be allocated and routed. This process is independent of the ISP or other authority that allocated the IP name space.
If changes are not made at the ISP or issuing Authority, or have not yet propagated, then this configuration will generate 'nslookup' and 'dig' errors or incorrect results. IPv6 reverse mapping uses the normal principle of reversing the address and placing the result, in the case of IPv6, under the domain IP6.ARPA.
The constructed domain name, however, does not have to reflect the address segmentation between the global routing prefix and the end-user part of the address as shown in the preceding example.
In the case of IPv6 reverse-mapping (and forward-mapping) may not be a simple process as discussed in the next section. Methods involving DHCPv6 may be configured to provide Dynamic DNS (DDNS) updates to the forward and reverse-mapped zones though in the case of very large networks even this can be problematic due to the potentially large number of hosts involved. These changes can only be mapped using DDNS since they are by nature both dynamic and transient.
It is worth noting however, that as with the equivalent IPv4 addresses, the only current applications which are known to use reverse mapping consistently are mail systems. The first part is essentially a simple IPv4 calculator and is invoked using the IPv4 Info button.
BIND is completely agnostic about line ends (CR+LF or LF) so notepad on windows, for example, will generate a useable zone file. The first part is essentially a simple IPv6 calculator and is invoked using the IPv6 Info button.
The default initial sequence number is 0 appended to the Host Label: however, this value may be changed by adding a value in Host No. Sometimes this is required for diagnostic purposes, more frequently these days it is used for security purposes to trace a hacker or spammer. As with all things when we understand what is being done and why - all becomes as clear as mud. This is a tad awkward and would make it impossible to construct a sensible tree structure that could be searched in a single lifetime. If the addresses were to change then the owner of the domain that maps these addresses would be able to make the necessary changes directly with either the relevant registrar i.e. The key difference from the IPv4 IN-ADDR.ARPA domain (described previously) is that a nibble is the unit of delegation. However, RFC 4941 also notes that a DNS entry defeats the object of privacy and instead recommends to either not register in the forward or reverse map, or to used some randomized host name if forced to do so by operational needs (some security systems do a reverse DNS lookup and reject failures).
Thus, steps should be taken to ensure that, at least, these hosts have a valid IPv6 reverse-map. The second part will generate a skeleton IPv4 reverse mapping file using the data supplied in the first part. In addition, the Delegation RRs (CNAME or DNAME) that should be added to the ISP's (or delegater's) reverse map file, as described in IPv4 Reverse Map Delegation, are generated in Delegation RRs:. If the tester does not work for you we are very, very sad - but yell at your browser supplier not us.
The second part will generate a skeleton IPv6 reverse map file using the data supplied in the first part.

If a mail server is missing a reverse DNS entry, other mail servers may reject mail from it.
You let them know what DNS servers are responsible for your domain names, and the registrar sends this information to the root servers (technically, the parent servers for your TLD).
Indeed, most modern mailing systems use reverse mapping to provide simple, first-cut, authentication using a dual look-up process - IP to name and name to IP.
In such configurations it is imperative that a reverse-map covering the range of private IP addresses be included in the local DNS configuration. Enter the name of the second NS server (a unqualified name or a FQDN) for this zone in NS 2: (for externally visible domains this is required, for private domains this is not essential, but is usually present through habit or for resilience).
These RRs can be copied and pasted directly into the appropriate place in the reverse zone file and do not include any ancilliary RRs (such as SOA, NS etc.). However, if you do think you have found an error, or want to suggest improvements, please take the time to email using the link at the foot of this page. Then, anyone in the world can access your domains, and you can send them to any IP addresses you want.
IPv4 reverse mapping is not mandatory though, as indicated by the mail example, it is essential for hosts that send mail, using either a Mail Transfer Agent (MTA) or a Mail User Agent (MUA). Failure to do so may cause local applications to fail or give strange results (none of them good) because the reverse-map request will be responded to by the DNS hierarchy where such private addresses are meaningless. Each byte (or octet) is comprised of 8 bits; a nibble is part of a byte and consists of 4 bits. You have full control over your domains, and can send people to any IPs (whether or not you have control over those IPs, although you should have permission to send them to IPs that are not yours).Reverse DNS uses a similar method.
In the case of IPv6 reverse-mapping was originally mandatory but as part of the seemingly relentless move to relax the goals of the original specifications (doubtless for good operational reasons) it is no longer a mandatory requirement. In order to limit the negative effect of such mis-configuration BIND introduced the empty-zones-enable statement which defaults to a state that will minimise damage to the DNS hierarchy. Edit the $ORIGIN statement in the resulting Reverse File: to reflect that defined in the appropriate DNAME RR.
However, BIND's default option does not solve the problem for local applications which may depend upon correct local results from a reverse-map query.
Enter the name of the second NS server for this zone in NS 2: (for externally visible domains this is required, for private domains this is not essential, but is usually present through habit or for resilience). You let them know what DNS servers are responsible for the reverse DNS entries for your IPs (or, they can enter the reverse DNS entries on their DNS servers), and your ISP gives this information out when their DNS servers get queried for your reverse DNS entries. It is, and remains, a serious mis-configuration of DNS to not include a reverse-may for all local (RFC 1918) addresses.
Click the Generate IPv4 Reverse Zone to activate and the results will appear in Reverse Zone.
Click the Generate IPv6 Reverse Zone to activate and the results will appear in Reverse Zone.
Without Step 2, nobody will be able to reach your DNS servers.If you can comprehend the above paragraphs (which takes some time), you'll understand the biggest problem that people have with reverse DNS entries. The biggest problem people have is that they have DNS servers that work fine with their domains (standard DNS), they add reverse DNS entries to those servers, and it doesn't work.
AND your ISP or bandwidth provider MUST set up the reverse DNS on their end, so that DNS resolvers around the world will know that your DNS servers are the ones to go to when looking up the reverse DNS for your IP addresses.

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