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

1.1 Domains and Subdomains

The entire Internet is divided into domains, i.e., name groups that logically belong together. The domains specify whether the names belong to a particular company, country, and so forth. It is possible to create subgroups within a domain that are called subdomains. For example, it is possible to create department subdomains for a company domain. The domain name reflects a host’s membership in a group and subgroup. Each group has a name affiliated with it. The domain name of a host is composed from the individual group names. For example, the host named consists of a host named bob inside a subdomain called company, which is a subdomain of the domain com.

The domain name consists of strings separated by dots. The name is processed from left to right. The highest competent authority is the root domain expressed by a dot (.) on the very right (this dot is often left out). Top Level Domains (TLD) are defined in the root domain. We have two kind of TLD, Generic Top Level Domain (gTLD) and Country Code Top Level Domain (ccTLD). Well known gTLDs are edu, com, net, and mil which are used mostly in the USA. According to ISO 3166, we also have two letter ccTLD for individual countries. For example, the us domain is affiliated with USA. However ccTLD are used mostly outside the USA. A detailed list of affiliated ccTLD and their details are listed in Appendix A.

The TLD domains are divided into subdomains for particular organizations, for example,,, Generally, a company subdomain can be divided into lower levels of subdomains, for example, the company Company Ltd. can have its subdomain as and lower levels like for its billing department, for its security department, and for its headquarters.

The names create a tree structure as shown in the figure:

Figure 1.1a: The names in the DNS system create a tree structure

The following list contains some other registered gTLDs:

  • The .org domain is intended to serve the noncommercial community.

  • The .aero domain is reserved for members of the air transport industry.

  • The .biz domain is reserved for businesses.

  • The .coop domain is reserved for cooperative associations.

  • The .int domain is only used for registering organizations established by international treaties between governments.

  • The .museum domain is reserved for museums.

  • The .name domain is reserved for individuals.

  • The .pro domain is being established; it will be restricted to credited professionals and related entities.