Book Image

DNS in Action

By : CP Books a.s.
Book Image

DNS in Action

By: CP Books a.s.

Overview of this book

The Domain Name System is one of the foundations of the internet. It is the system that allows the translation of human-readable domain names into machines-readable IP addresses and the reverse translation of IP addresses into domain names. This book describes the basic DNS protocol and its extensions; DNS delegation and registration, including for reverse domains; using DNS servers in networks that are not connected to the internet; and using DNS servers on firewall machines. Many detailed examples are used throughout the book to show perform various configuration and administration tasks.
Table of Contents (16 chapters)
DNS in Action
About the Authors
Country Codes and RIRs

1.3 Reverse Domains

We have already said that communication between hosts is based on IP addresses, not domain names. On the other hand, some applications need to find a name for an IP address—in other words, find the reverse record. This process is the translation of an IP address into a domain name, which is often called reverse translation.

As with domains, IP addresses also create a tree structure (see Figure 1.2). Domains created by IP addresses are often called reverse domains. The pseudodomains inaddr-arpa for IPv4 and for IPv6 were created for the purpose of reverse translation. This domain name has historical origins; it is an acronym for inverse addresses in the Arpanet.

Under the domain, there are domains with the same name as the first number from the network IP address. For example, the domain has subdomains 0 to 255. Each of these subdomains also contains lower subdomains 0 to 255. For example, network belongs to subdomain This actual subdomain belongs to domain, and so forth. Note that the domains here are created like network IP addresses written backwards.

Figure 1.2: Reverse domain to IP address

This whole mechanism works if the IP addresses of classes A, B, or C are affiliated. But what should you do if you only have a subnetwork of class C affiliated? Can you even run your own name server for reverse translation? The answer is yes. Even though the IP address only has four bytes and a classic reverse domain has a maximum of three numbers (the fourth numbers are already elements of the domain—IP addresses), the reverse domains for subnets of class C are created with four numbers. For example, for subnetwork we will use domain It is as if the IP address suddenly has five bytes! This was originally a mistake in the implementation of DNS, but later this mistake proved to be very practical so it was standardized as an RFC. We will discuss this in more detail in Chapter 7. You will learn more about reverse domains for IPv6 in Section 3.5.3.