FQDN

Stands for "Fully Qualified Domain Name." An FQDN is a the most complete domain name that identifies a host or server. The format is:

[hostname].[domain].[tld].

For example, "www.techterms.com." is an FQDN since it contains a hostname ("www") and a domain name ("techterms.com"), followed by a trailing period. The name "techterms.com" is not fully qualified because it does not include a hostname or end with a period.

An FQDN can be broken down into four parts:

1. Hostname: www, mail, ftp, store, support, etc.
2. Domain: apple, microsoft, ibm, facebook, etc.
3. Top level domain (TLD): .com, .net, .org, .co.uk, etc.
4. Trailing period: the final period in an FQDN indicates the end of the name, implying the previous string is the TLD.

Hostname and Domain Name

The domain and TLD comprise the domain name, while the hostname specifies different services and protocols for the domain. For example, "mail.example.com" is often the required format when configuring the SMTP server for an email account. The server address "ftp.example.com" is commonly used when connecting to an FTP server. Name servers typically use the naming convention "ns1.example.com" and "ns2.example.com".

The hostname may also specify a website subdomain. The most popular subdomain is "www." Other common subdomains include "support," "dev," "store," and "forum." Some sites use variations of "www" such as "web", "ww1," "www2," etc.

The Trailing Period

Technically, a fully-qualified domain name includes a trailing period, which indicates the end of the name. Since a hostname can include multiple subdomains, such as "en.support.example.com," it is more reliable to process an FQDN backwards, beginning with the TLD and ending with the hostname. Ironically, the period serves as the starting point when a computer processes an FQDN.

While the trailing dot is part of a fully-qualified domain name, in most cases, it is implied. For example, you don't need to enter the period when typing in a web address in the address bar of your web browser or when entering the mail server in your email client. In other places, such as a DNS zone file, it is important to include the trailing period for each FQDN.

NOTE: A domain name that includes the domain, TLD, and period, but no hostname is called a partially-qualified domain name, or PQDN.