There are meanings or rules for every part of the domain name. The descriptor at the right most part is the top level domain name or TLD. Each descriptor is called a domain and is separated with dots or periods. Top level domain names can be generic domains or country specific domains. Middle name or second or third level domains usually are reserved for the end-users who register the names in the DNS registry. If you are thinking of starting a business with a certain name, it is better to register your domain name well ahead in DNS registry. Apart from identifying a server it also allows a resource to be moved physically locally or globally in the network with corresponding IP address change and corresponding change in the domain names registry.
In domainname.com .com is the generic top level domain name. It can be one of the more than 20 generic names such as .com, .net, .edu, .gov, etc. generic names or one of the many country specific domain names such as .in,.uk,.de,etc. It is the two letter country abbreviation. Domainname is the second level identifier or subdomain. Top level domain names are managed by the International Corporation for Assigned Numbers (ICANN). There can be third, fourth or even more sub domains and there is no limitation, but there is a limit to the number of characters allowed in domain names and it is limited to 253 characters.
It is very common to mistake the domain name for URL (Uniform Resource Locator), but domain names are not URL but domain names do appear in the URL. The main purpose of a domain name is to give a name to the internet server that can be easily remembered and recognized. Another thing to remember is that domain name and host name are also not the same thing. A host name has to have an IP address associated with it and it is always a domain name but not the other way around.
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