TCP / IP For Dummies
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Normal DNS queries are forward lookup queries, they request the IP address that corresponds to a fully qualified domain name. A reverse lookup is the opposite of a forward lookup: It returns the fully qualified domain name of a host based on its IP address.

Reverse lookups are possible because of a special domain called the domain, which provides a separate fully qualified domain name for every possible IP address on the Internet.

To enable a reverse lookup for a particular IP address, all you have to do is create a PTR record in a reverse lookup zone (a zone that is authoritative for a portion of the domain). The PTR record maps the domain name for the address to the host’s actual domain name.

The technique used to create the reverse domain name for a given IP address is pretty clever. It creates subdomains beneath the domain by using the octets of the IP address, listing them in reverse order. For example, the reverse domain name for the IP address is

Why list the octets in reverse order? Because that correlates the network portions of the IP address (which work from left to right) with the subdomain structure of DNS names (which works from right to left). The following description should clear this up:

  • The 255 possible values for the first octet of an IP address each have a subdomain beneath the domain. For example, any IP address that begins with 207 can be found in the domain.

  • Within this domain, each of the possible values for the second octet can be found as a subdomain of the first octet’s domain. Thus, any address that begins with 207.126 can be found in the domain.

  • The same holds true for the third octet, so any address that begins with 207.126.67 can be found in the domain.

  • By the time you get to the fourth octet, you’ve pinpointed a specific host. The fourth octet completes the fully qualified reverse domain name. Thus, is mapped to

As a result, to determine the fully qualified domain name for the computer at, the client queries its DNS server for the FQDN that corresponds to

About This Article

This article is from the book:

About the book authors:

Candace Leiden consults on systems and database performance and instructional design for international courseware. Marshall Wilensky is a retired consultant and network manager for multiprotocol networks.

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