Domain Name Basics

Before we discuss domain names, we need to understand Internet address. Internet Address also known as IP address (version 4) is a 32-bit numeric address written as four 8-bit numbers separated by dots. Each 8-bit number represents a number between 0 and 255. An IP address is numerical address used by computers to uniquely identify a computer that are interconnected by the Internet. An IP address is a globally unique address that anyone in the world can find your computer.

The Internet Address is composed of numbers that the computers can understand, but they are hard for humans to remember. To make Internet address human friendly, domain names are introduced. A domain name is mapped to an IP address by a domain name system (or DNS), so that it is easily understood by both humans and computers. The domain names and Internet addresses are administered by Internet Assigned Numbers Authority.

The domain name is a human readable Internet Address. To setup a web site or create an email address, you'll need a domain name. A domain name is comprised of at least two parts: top level domain (TLD) and second level domain (SLD). The TLDs are administered by IANA, and they are pre-determined. The SLDs are domain names that an individual or an organization can register through a domain registrar for it's own use. The third, fourth and (so on) level domain names are arbituary domain names that are assigned by the SLD owners to further organize its domain name.

Top Level Domains

Top level domain names are divided into three categories: generic TLD (gTLD), country code TLD (ccTLD) and infrastructure TLD. The generic top-level domains are the ones most frequently used on the web, and they are as follows:

The ccTLDs are reserved for countries and their dependent territories. Each two letter contry code is designated to an individual country. A list of all country codes can be found in Root-Zone Whois Information page of the IANA website.

The infrastructure TLD is limited to ".arpa" domain name, and is used execlusively for Internet infrastructure purposes. The .ARPA is the Address Routing Parameter Area domain, and is commonly used for reverse domain name resolution purpose.

Second Level Domains

The second-level domain names (SLDs) in combination with top-level domain name (TLD) are the ones that can be registered through ICANN acredited registrars. The domain name is registered for a period of time between 1 year and up to a maximum of 10 years. Think of domain name registration as a right to use the name for a specific period of time (as in lease), and as long as you renew before its expiration, the name belongs to you.

Domain names must be at least two characters long and no more than 63 characters maximum, excluding the top-level domain name. The characters can include any combination of letters, numbers or hyphens. The first and last character cannot be a hyphen. Domain names are not case-sensitive.

Third Level Domains (and beyond)

The third (fourth, and ...) level domain names are also known as subdomain (or canonical) names. A subdomain is part of a domain name created by the SLD owners to create additional names (addresses). A purpose of creating additional subdomain names is to create globally unique addresses within its organization so that they can be accessed globally. The most common subdomain name is www, as in Creating additional subdomains allow an organization to make more computers available on the Internet (remember, the domain name is a human friendly Internet Address) by creating globally unique addresses.