What is a fully qualified domain name (FQDN)?
A fully qualified domain name (FQDN) is the complete address of an internet host or computer. It provides its exact location within the domain name system (DNS) by specifying the hostname, domain name and top-level domain (TLD). For example, for the domain name www.whatis.com, "www" is the hostname, "whatis" is the domain name and ".com" is the top-level domain.
DNS filtering and network rules can be added to firewalls to allow or deny traffic to specific computers or domains.
Are FQDN and URL the same?
In internet communications, the terms FQDN and URL are sometimes used interchangeably. However, an FQDN isn't the same as a URL but rather is a part of it that fully identifies the server to which the request is addressed.
An FQDN doesn't carry the TCP/IP protocol information -- such as Hypertext Transfer Protocol (HTTP) or Hypertext Transfer Protocol Secure (HTTPS) -- which is always used at the beginning of a URL. Therefore, adding the prefix http:// or https:// to the FQDN turns it into a full URL. Also, URLs can specify directory paths, file names and TCP port numbers, which FQDNs don't include.
When FQDNs are provided in place of full URLs to access resources on the internet, the protocol by default is assumed to be HTTP or HTTPS.
Examples of an FQDN
For a device to be accessible on the internet, it needs a fully qualified domain name. For an FQDN to be complete, it must meet the following criteria:
- The entire FQDN has a max length of 255 characters.
- It uses only letters, numbers and hyphens.
- A number or letter must take the first position on each label.
An FQDN is always labeled in the format of hostname.domain.TLD. For example, a mail server on the yahoo.com domain might use the FQDN mail.yahoo.com.
The following are some examples of an FQDN:
Note that each label -- including hostname, domain name and TLD -- is separated by a period and ends with a trailing period. However, most systems simply imply the period, even if one isn't provided. For example, DNS servers consider www.microsoft.com an FQDN.
Why and when to use an FQDN
The following are popular use cases for FQDNs:
- Connecting to a remote host. When trying to connect to a remote server, the DNS server performs a lookup in its DNS table to resolve the FQDN to its corresponding IP address. For example, if a connection is attempted to a remote server with only the hostname, such as example instead of example.com, the DNS table returns an error message. Sometimes, using an FQDN is essential to reach a remote server, especially if it doesn't share the same internet service provider.
- Configuring the DNS and IP address. FQDNs are easier to remember than IP addresses and are needed to configure the DNS and IP address of a device on the internet. For example, when trying to reach Google, it's much easier to type google.com in the browser, instead of finding and typing its numerical IP address.
- Getting an SSL certificate. Each FQDN is issued a secure sockets layer (SSL) certificate to encrypt the connection between the browser and the web server and to specify its location identity. Without an FQDN, SSL services can't be used properly.
- Migrating to a new server. An FQDN can be beneficial when migrating services to a different server. Using an FQDN instead of an IP address quickly changes the DNS records and prevents communication issues and outages that can happen when changing an IP address.
- Branding potential. An FQDN lets site visitors know the identity, location and presence of a website, which can help with brand promotion.
- Search engine optimization (SEO). Having an FQDN can improve a website's SEO rankings in the web browsers.
- Accessing domain services. FQDNs are useful in connecting to domain services, such as email or file transfer protocol. For example, the FQDN of a mail server is needed to connect a domain name's email to an email application on a phone, such as Gmail or Apple Mail.
How to find an FQDN
FQDN use isn't limited to the internet, but each PC and server has its own FQDN. It's easy to find the FQDN of any device, depending on which operating system is being used.
Finding the FQDN for Windows 11
Use the following steps to find an FQDN for Windows 11:
- Open the Control Panel.
- Click System and Security> System.
- Select the See the name of this computer
- The Full device name is listed under Device specifications.
Finding the FQDN for macOS
Use the following steps to find an FQDN for macOS:
- Select the Apple menu, System Settings and click General in the sidebar.
- Click About on the right. You might need to scroll down.
- 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. For Linux, opening the terminal and entering hostname -A, where the A is case-sensitive, into the prompt returns the FQDN.
What is a partially qualified domain name (PQDN)?
A PQDN is part of an FQDN that isn't fully specified. In general, a PQDN gives an incomplete piece of information regarding where a resource is located on the internet or on a private server. For example, a hostname of p402srv01 is classified as a PQDN, because it only tells the hostname. The domain that it belongs to is unknown.
PQDNs are typically used for convenience and are applicable where the full name isn't required to resolve the host, as the domain might already be known elsewhere. In those special scenarios, only the hostname is required for a particular task.
To understand the difference between a PQDN and an FQDN, think of a phone number, where the PQDN is the three-digit area code and the FQDN is the 10-digit phone number.
The process of name resolution plays an integral role in network communications. Learn how the name resolution process is used for resolving hostnames, querying servers and locating IP addresses.