Q. What is a FQDN?
A. This is an acronym for a "Fully-Qualified Domain Name." Within an organization, it's convenient to refer to a computer by the first part of its name, knowing that "everyone" will know that the remainder is the same as the other computers in the organization. Thus, you may speak of "sneezy" and "dopey", knowing that they're really two computers at "seven-dwarves.org".
But computers need the fully written-out name (the FQDN), such as "sneezy.seven-dwarves.org." or "dopey.seven-dwarves.org." to identify a computer. Most user software has the ability to add a "search domain" to all partially-qualified domain names, filling out the missing part of the FQDN. But some DNS servers require the FQDN to work properly with InterMapper. To be safe, it's always correct to enter the full domain name.
Technically, a FQDN requires a "." at the end. Just as the search domain is tacked onto the end of a partial domain name, most user software adds the trailing "."
Tip: Even though you enter a FQDN when specifying a computer, you can use the Short, Smart Name when constructing a label for a device.