First of all, please read this short description of the domain.

A root domain is superior to a subdomain and its name cannot be divided by a dot.

1. When you create a domain at your domain management account (your domain provider should create such account for you), the system will suggest a name without a dot. 

For example, the name can be something like mydomain.com but it can not look like my.domain.com . Domain providers system would block a possibility of creating such root domain.
Why does it happen?

The reason is that a dot separates a subdomain (e.g. my ) from a root domain (the part of the name that goes after a dot .domain.com . In other words, an address my.domain.com  is a subdomain of domain.com the root domain.

2. Usually, you should be able to create an unlimited number of subdomains on your domain management account. It means, that for a root domain domain.com you can create various subdomains, e.g. my.domain.com , your.domain.com ,our.domain.com  and so on. Creating subdomain usually costs less and does not require creating new accounts at your domain provider's. It should be enough just to have a single account where you already have a root domain.

If you are not sure how to create subdomains, please contact your domain provider.

3. As you may have noticed, none of above examples start with www. , because as a matter of fact www  is a subdomain added to a root domain. In most cases servers handle www subdomain automatically, and it is considered that it directs to a root domain. Therefore it is not necessary to create DNS records for domain like www.domain.com .

4. A landing page can be redirected to both root domains and subdomains. But why does it matter? Please take a look at below example.

Mr X has launched a business, "Sweet" confectionery. He bought a domain sweet.com  and created a website for his business. The customers can check the opening hours, the offer and confectionery localization. Later on, Mr X decided to organize a special offer for his clients.
He created a landing page containing all the necessary information regarding the special offer, and then redirected his domain sweet.com to this landing page.
As a result, the customers entering sweet.com or www.sweet.com were only able to see the special offer landing page, but the main website of the confectionery has been completely replaced. That is why Mr X should have created a subdomain like specialoffer.sweet.com and publish his landing page on it. In that situation the main website on domain sweet.com  would still work normally, and the clients could check information about the special offer on www.specialoffer.sweet.com  or specialoffer.sweet.com subdomains.

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