The top-level domain or TLD name is what every domain name ends with. It is almost always either one of a small list of generic names consisting of two or more characters or a two character territory code. They are also called first-level domains. The second-level domain or SLD names are the names directly to the left of top-level domains. Immediately to the left of SLD names are third-level domains and so on. Each level is separated by a dot or a period symbol and virtually has no limitation.

The second level domain is usually the name of the company or the name used in the Internet while the third level is used to designate a specific host server. Since multiple servers can serve a single sub-domain, it is possible that a sub-domain may not have a real purpose.