(SMTP Commands - Reference)

Fully Qualified Domain Name (FQDN)

The two SMTP commands HELO and EHLO can take a fully qualified domain name as an argument. A fully qualified domain name (often shortened to FQDN) is a complete and unique domain name for a specific computer / host connected to a network or the Internet. Fully qualified domain names are sometimes also called absolute domain names.
A FQDN consists of a hostname and domain name. For example, www.samlogic.net is a fully qualified domain name where www is the hostname and samlogic.net is the domain name. The rightmost part in the domain name (.net in our example) is called a top level domain. See the picture below:

A fully qualified domain name (FQDN)
Fully qualified domain names are necessary to give every computer / host a guaranteed unique human-readable name / location in the tree hierarchy of a Domain Name System (DNS). By using fully qualified domain names there is never a need to specify IP addresses in the communication between different computers / hosts. This will make life much easier for humans because it is much easier to remember and recognize a domain name compared to an IP address. IP addresses are still used but beneath the surface. A DNS server is used to match a domain name to an IP address.

Wikipedia - Fully qualified domain name
Wikipedia - Domain Name System (DNS)

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