In website addresses, a fully qualified domain name (FQDN), also called an absolute domain name, is a domain name that identifies all levels of the domain, including a subdomain in addition to the usual root, top-level domain (TLD), and second-level domain, divided by periods called "full-stops" or "dots." The domain names "taxes.state.mn.us." and "mail.google.com." would both count as fully-qualified domain names. A fully qualified domain name must also have a period at the end. Besides the period at the end, most fully qualified domain name structures consist of three parts, though some top-level domains, like the United Kingdom top-level domain .co.uk, have two parts, which gives some fully qualified names four domain parts.
Usually found where the "www" is found in many domains, the subdomain, known sometimes as a third-level domain, is one part of the domain that makes it a fully qualified domain name. A subdomain points to a folder on a server where a specific site is hosted. On "www.google.com," a subdomain pointing to the web server is "mail.google.com." When a user does not use her own domain name, the website is often kept at a subdomain address of the web host's server. The difference between a web address with a subdomain and a fully qualified domain name is a period at the end of the domain name, which stands for the root of the domain.
Starting from the right end of a domain address, a top-level domain is the first component of a fully qualified domain name. This part can give a user information about the geographical origin of the site or the type of group running the website. The most common top-level domains, called generic top-level domains (gTLD) include .com, .org. and .gov domains.
Second-level domain names are considered to be subdomains to top-level domains. They are usually the second part of the address from the right, except in cases when the top-level domain is two parts and the second-level domain is the third domain part from the right. An example of a second-level domain would be the word "wiseGEEK" in www.wiseGEEK.com.
A domain name is the text address a user types into the address bar of a browser to navigate to a website. Though a domain name is used to get to the website, the actual address for the website is really a series of numbers. Behind the scenes, the domain names is contained in a directory with other domain names, where the numerical address for the website is kept. Usually, domain names relate to either the title or the subject matter of the website. The system that directs web traffic to the correct numerical address based on domain name is called the domain name system.
Domain levels are part of a domain name system (DNS), which contains protocol for naming domains. The domain name system translates the domain names into the numerical addresses understood by computer hardware. Without DNS, a user would be required to enter a number for an address instead of a domain name.