Domain names are organized hierarchically into domains. For example, the name www.namesurfer.com belongs to the namesurfer.com domain, which is a subdomain of the com domain, which in turn is a subdomain of the root domain.

Domain names and domains are analogous to file names and directories in a tree-structured file system - they just happen to use dots instead of slashes or backslashes as separators, and their components are written in the opposite order.

Each domain may be administered as a separate unit (a zone), which may include none, some, or all of its subdomains.

The dot-separated components of a domain name are known as "labels". Each label may be up to 63 characters in length, and the entire domain name is restricted to 255 characters.

Although the DNS itself does not restrict the character set of domain labels, other software that uses domain names may have such restrictions. To avoid problems, it is recommended that only letters, digits, and minus signs (-) be used, particularly in domain names that have A or MX records.