In the very most cases DNS is used, in order to determine to a Domain name the associated IP address. In addition, there is the reverse situation, with which for a given IP address the name is needed. DNS inquiries, with which to a IP address the name is to be determined, become as inverse Anfragen1) (also: inverse Requests) designates.

Since it would be extremely time-consuming, with an inverse inquiry the entire domain tree for the desired IP address to scan (it is not well-known, in which branch the looked for entry is), its own domain for inverse accesses was formed, in ADDR ARPA domain. Below this domain only three levels exist, so that maximally three steps are necessary for the dissolution of a IP address.

The direct of in ADDR ARPA has a number between 0 and 255 as label and to represent the first component of a IP address. (Examples: 10. in ADDR ARPA or 193.in-addr.arpa).

The next level in the tree represents the second component of a IP address (example: 17.171.in-addr.arpa contains the IP addresses 171.17.x.y) and the lowest level finally the third component (example: 155.39.194.in-addr.arpa contains all well-known IP addresses the net 194.39.155.0 /24 - thus e.g. 194.39.155.69).

As is shown by the examples, a reverser name contains the IP address components in reverse order. This structure makes a refining possible of the reversen address area in several steps. So for example the net 171.17.0.0 /16 first by the reverse domain 17.171.in-addr.arpa can be represented. All again assigned IP addresses from this segment are registered there. At a later time (e.g. 11.17.171.in-addr.arpa) can be put on. Inverse inquiries to IP addresses, for which no exists, are dissolved thereby further over the global domains.

Reverse domains function exactly the same as normal. That applies essentially also to the appropriate zone files (see: Zone). At the beginning of a reversen domain assigned zone file a SOA resource record stands for one or more LV followed from resource record. As further RR-types however only PTR resource record is permissible. With a PTR-RR stands on the left of a IP address and on the right of a name - contrary to A resource record, where on the left of a name and on the right of a IP address stands.

Example:

1.11.17.171.in addr.arpa. 1285 IN PTR server1.example.com. 

The corresponding entry of the domain example.com looks then as follows:

server1.example.com. 1800 IN A 171.17.11.1 

1) Note to the linguistic usage: reverse refers to the fact that in reverse order in in ADDR ARPA domain abliegen.inverse refers the components of a IP address to the DNS access: The reverse information is requested - a name and not as usual a IP address.

See also

• Inverse search

Related links

• Reverse DNS

Articles in category "Reverse domain"

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