The FQDN (Fully Qualified Domain Name) is an entire domain name address, including hostname, domain name and top-level domain.
An example of an FQDN: www.accuwebhosting.com is a fully qualified domain name where www is the host, accuwebhosting is the second-level domain and .com is the top level domain
The FQDN is required any time you reference a domain name outside a zone, optional when referencing names inside the zone. CNAME records, NS records, and MX records might refer to the names outside the zone, they often required FQDN.
