What Is a Fully Qualified Domain Name (FQDN)?

The FQDN serves to show the exact location of a computing resource inside the Domain Name System (DNS) hierarchy. An FQDN is traditionally written as a list of domain labels: the top-level domain, the second-level domain name, a subdomain (if used), and the host domain, each separated by dots or periods. The label hierarchy is read from right to left. As an example of FQDN syntax, in the address www.shop.example.com the “.com” is the top-level domain, “example” is the second-level domain name, “shop” is the subdomain, and the “www” is the hostname.

• The top-level domain (TLD) is the highest level in the hierarchical structure of domain names and represents the most general categorization of the domain. In addition to .com, other common generic TLDs include .org, .net, and .info. Country code TLDs are also used: .ca (Canada), .us (United States), .de (Germany).
• The second-level domain (SLD) is located immediately to the left of the TLD. The SLD is typically chosen by the owner or administrator of the domain and is often associated with the name or brand of the organization, website, or service. The combination of the second-level domain and the top-level domain creates the root domain, a distinctive and recognizable address for accessing websites and other Internet resources.
• Subdomains are used to organize and categorize different sections or services within a domain and provide a way to create distinct namespaces and hierarchical organizations under a primary domain. Subdomains are often used to represent distinct services, departments, or sections of a website. For example, “shop.example.com” could be the address of an online store.
• The hostname is the leftmost element of a domain name. Hostnames can be used for a variety of purposes, such as identifying web servers ("www"), mail servers ("mail"), ftp servers ("ftp"), and more. For instance, www.example.com is the address of the web server associated with the domain example.com. Hostnames provide a way to organize and categorize different services within a domain.   
• The Internet Corporation for Assigned Names and Numbers (ICANN) manages the assignment and administration of domain names and TLDs to ensure the stability and security of the global domain name system.

Traditional domain names are based on the ASCII character set, which includes the Latin alphabet and a limited set of characters. However, many languages around the world use scripts with characters that are not represented in ASCII. International domain names (IDNs) address this limitation by allowing the registration and use of domain names in local languages and scripts.

IDNs are domain names that include characters from non-ASCII character sets, such as letters with diacritics or characters from non-Latin scripts. IDNs enable the use of domain names in languages other than English and support the inclusion of characters from various scripts and writing systems, promoting inclusivity and linguistic diversity on the Internet.

IDNs use Unicode, a standardized character encoding system, to represent non-ASCII characters. Unicode assigns unique numeric values to characters and symbols from virtually all the world's writing systems, allowing domain names to include characters from various scripts, such as Cyrillic, Arabic, Chinese, Japanese, Korean, and many others, beyond the limited set of characters available in the ASCII character set.

In turn, another encoding system called Punycode converts the non-ASCII IDNs into ASCII-compatible representations. This allows domain names containing characters from various scripts to be represented in a format that is compatible with the ASCII character set, which is traditionally used for domain names in the DNS. 

• Trailing dot: In a FQDN, a trailing dot (for instance, “www.example.com.”) represents the root of the DNS hierarchy. The dot (or period) at the end serves to terminate the domain name, making it an absolute, fully qualified reference. When an FQDN is specified without a trailing dot, the DNS system automatically appends the root domain to the name. For example, "www.example.com" is equivalent to "www.example.com." However, the presence or absence of the trailing dot can influence the interpretation of the domain name in certain situations. When specifying domain names in DNS configurations, zone files, or other technical contexts, including the trailing dot is important for accuracy and proper interpretation. 
• Wildcard subdomains: A wildcard subdomain refers to a subdomain that is specified using the wildcard character * (asterisk) as a placeholder for any arbitrary subdomain name. Hence, “*.example.com” represents any subdomain under the domain "example.com." Wildcard subdomains are often used in situations where subdomains are created dynamically or where the exact list of subdomains is not predefined. A wildcard subdomain can also serve as a catch-all rule for any subdomain that matches the specified pattern. This can simplify DNS configurations by allowing a single rule to cover multiple subdomains.

According to ICANN, the terms "Fully Qualified Domain Name" and "Absolute Domain Name" are often used interchangeably, and in most contexts, they refer to the same concept. Both terms describe a domain name that includes the full hierarchy of labels needed to specify the exact location of a resource in the DNS.

A PQDN is a domain name that lacks the complete hierarchy of labels necessary to make it a FQDN; a PQDN does not provide the full path needed to locate a specific resource on the Internet. Commonly, a PQDN lacks the hostname, so while www.example.com is that site’s FQDN, “example.com” would be that site’s PQDN. Most website developers configure their site’s DNS to redirect visitors to the same page whether they key in the FQDN or PQDN.

A FQDN is a complete domain name that specifies the exact location of a resource in the DNS, while a URL refers to the full address for accessing resources on the Internet, including the protocol and additional components beyond the domain name. URLs are commonly used in web browsers to navigate to specific web pages.

FQDNs serve a number of important functions, including: 

• Identifying network resources. FQDNs are used to uniquely and exactly identify network resources, such as servers, routers, and devices, on the Internet. Each resource can be assigned a distinct FQDN to facilitate precise locating and communication.
• Email addressing. In an email address, the domain name is represented by the site’s FQDN. For instance, in the email address "user@example.com," "example.com" represents the mail server's domain.
• Web server hosting. FQDNs are used in web hosting to identify and access web servers. Websites are typically associated with FQDNs, allowing users to access specific web pages by entering the complete address (for instance, "www.example.com"). This is critical for hosting multiple websites on a single server, each with its own FQDN.
• DNS resolution. DNS translates human-readable domain names into IP addresses, enabling computers to locate and communicate with each other on the Internet. FQDNs are fundamental for DNS resolution, as they are used in DNS queries and responses, allowing systems to resolve hostnames to their corresponding IP addresses.

FQDNs play a crucial role in identifying, connecting, and securing network infrastructure, communication, and web services. These uses include: 

• Obtaining an SSL/TSL certificate. FQDNs are instrumental for securing web communication through SSL/TLS certificates, which protect connections between a web server and a browser. SSL/TLS certificates are typically issued for specific FQDNs to  ensure secure and encrypted communication between clients and servers.
• Remote host connections. FQDNs are commonly used in remote access scenarios, such as remote host and virtual machine connections. They contribute to the authentication process, ensuring accurate identification of and connection with the intended server.
• Accessing domain services, protocols, and remote servers. FQDNs are essential for many applications such as accessing FTP (File Transfer Protocol) servers and in the configuration of email services. FQDNs are also used to securely identify and authenticate hosts for Secure Shell (SSH) connections and to specify intended host servers in Telnet connections. For instance, within an FTP session, the FQDN may be used to specify the full path to a directory or file on the FTP server. Similarly, when sending emails, mail clients use the FQDN of the SMTP server to route outgoing messages and ensure proper delivery to the intended mail server.
• Migrating to a new server. Migrating services to a new server or infrastructure usually requires changes in IP addresses. Using FQDNs allows users to update DNS records to reflect the new IP addresses without requiring changes on client devices. FQDNs provide an abstraction layer, allowing users to refer to services by their domain names rather than specific IP addresses.

How to find your computer’s FQDN depends on your operating system.

1. Open the Control Panel.
2. Click System and Security> System.
3. Select the See the name of this computer
4. The Full device name (i.e., the FQDN) is listed under Device specifications.

1. Select the Apple menu, System Settings and click General in the sidebar.
2. Click About on the right. You might need to scroll down.
3. The full computer name or the FQDN appears at the top of the About settings.

Alternatively, in macOS, opening the terminal and typing hostname -f into the prompt returns the FQDN.

Opening the terminal and entering hostname -A, where the A is case-sensitive, into the prompt will return the FQDN.

DNS enables users to access services, making it one of the most important components in the network infrastructure. If DNS is unavailable, services won’t function properly. F5 BIG-IP DNS and F5 Distributed Cloud DNS offer unmatched DNS performance, managing query responses with multicore scalability and handling spikes in DNS query volume to make sure apps are highly available—even between multiple instances and across hybrid environments. F5 DNS platforms offer simple management, with easy visibility and programmability, ensuring that network architectures are easier to maintain and providing faster web browsing and reduced latency for improved user experiences.