In the example above "mydomain.com" is the root domain. There's nothing at the beginning before "mydomain.com", like "www" or "blog" or "shop".
The "www" you see in the image above is what is called a subdomain of mydomain.com. Subdomains can be created later, but you can't register a domain with them.
Technically, a domain name is read from right to left (which can seem backwards).
- It starts with a "." (which is usually not displayed).
- Then comes the so-called "top level domain" (TLD, e.g. .COM or .NET or .PIZZA).
- It is followed by what we call the "root domain", which is the bit that you can register and which is assigned to you as long as you keep the registration, e.g. "mydomain" in the example above.
- Anything after the root domain we call "subdomain". You can create as many subdomains for your domain name as you like. Up to the maximum length of a domain name, which is 253 characters in total. However, each 'level' may not be longer than 63 characters.
E.g., with "mydomain.com" something like "this.is.a.subdomain.of.mydomain.com" could be created.