What’s a Fully Qualified Domain Name (FQDN) and what’s it good for?

6 min read
Natalie Cullings

In the great landscape of domain names, not every domain name carries a full set of identifying credentials. You either have a Fully Qualified Domain Name (FQDN) or a Partially Qualified Domain Name (PQDN). So what’s the difference? We’ll start by covering what a Fully Qualified Domain Name is before we tackle how we look up the FQDN of a server and when we would need this information.

The one and only Fully Qualified Domain Name

A Fully Qualified Domain Name (FQDN) is the complete domain name of a specific computer, or host, online. An FQDN comprises of several elements: a hostname and a domain name. These elements are separated by a period.

All FQDN follow the format [hostname].[domain].[tld], where the domain might also include a subdomain. We read the FQDN from right, its root, to far left, its host.

Take www.GoDaddy.com, for example. The first element and domain level of the FQDN is the top level domain (TLD), which, in this case, is “.com.” Within the TLD, “GoDaddy” is the assigned domain name/second-level domain. Lastly, “www.” is the hostname.

Fully Qualified Domain Name Example

A hostname often specifies a particular service or protocol for a domain such as “mail” or “ftp” in “mail.domain.com” or ftp.domain.net, respectively. Many websites today do not include “www.” in their URLS, and therefore are only partially qualified domain names.

Technically, FQDNs contain an empty element to the right of the TLD that signifies the unnamed domain root zone, and thus a trailing period follows the TLD (www.GoDaddy.com.). However, today’s software (including internet browsers) usually processes the trailing period for us. The unnamed domain root zone essentially represents the internet.

A Fully Qualified Domain Name designates the specific location of an object within the Domain Name System (DNS) hierarchy; it communicates the host’s position relative to the root of the DNS namespace. An FQDN enables each entity connected to the internet (computer, server, etc.) to be uniquely identified and located within the internet framework.

Think of the DNS as the address book of the internet, which locates and translates domain names into IP addresses. A Partially Qualified Domain Name tells the DNS a general, incomplete piece of information about where a resource on the internet, or on a private server, lives. A Fully Qualified Domain Name provides a distinct, precise location within the DNS that we can access.

Look at it this way — if a PQDN is an area code, an FQDN is your ten digit phone number. If a PQDN is your home city, an FQDN is your home address.

How do you look up a Fully Qualified Domain Name?

Looking up the FQDN of your computer or server is simple. Just follow the instructions for your operating system below. If your machine does not provide the FQDN, it is not connected to a domain.

Windows 10. Within the taskbar’s “Search Windows” box, type “control panel” and select “system and security.” Next, select “system” and the FQDN is listed next to the Full Computer Name label.

Mac OS. Open terminal, and enter “hostname -f” into the prompt. Terminal will return the FQDN.

Linux. Open terminal and enter “hostname -A” into the prompt. The “A” is case sensitive. Terminal will return the FQDN.

Once you know your Fully Qualified Domain Name, you can make your device available online through the DNS.

FQDN Use Cases

Fully Qualified Domain Name Website
Photo: medithIT via Visual hunt / CC BY

In general, you will require an FQDN to make a computer, device, entity, etc. accessible on the internet; however, defining an FQDN locally isn’t sufficient to bring it online. You have to update the DNS record in the DNS settings so the DNS knows the specific location of that specific device.

Defining your DNS namespaces allows the DNS to connect the FQDN to IPs, and therefore locate your device online. If you’re interested in an introductory to the Domain Name System and how this process works, give this article a read.

Fully Qualified Domain Names have a plethora of applications since they are a fundamental component of the internet’s organization. Below, let’s break down a few instances in which you’ll want to have an FQDN handy.

Obtaining an SSL Certificate

SSL certificates are small data files that bind together a domain and server name, or hostname. They are installed on a server to tie an organization’s identity to its location. For this reason, they are only granted to a Fully Qualified Domain Name.

Most legitimate or major websites today have an SSL certificate(s), which activates the padlock and https:// protocol on a website.

SSL certificates are increasingly common, and even expected, when conducting monetary transactions and the transfer of other sensitive information.

Connecting to a host remotely

If you don’t want to, or can’t, connect to a remote host or virtual machine (VM) via raw IP, through SSH, for example, you need to specify the Fully Qualified Domain Name. The DNS server looks at its DNS table and resolves the FQDN to its IP address. Next, the server contacts the host and returns a login prompt.

If you’re using an application to connect to a server, you might not be able to resolve the hostname without the domain info.

In this case, you’ll need the FQDN so the DNS can find the server. If you’re connecting to a remote host that isn’t local to your ISP, the Fully Qualified Domain Name will likely be required.

Access a Particular Domain Service or Protocol

Essentially any activity that transfers info across a network involves the DNS, and therefore an accessible FQDN. If you’re connecting to a File Transfer Protocol (FTP) server or an email server, you will need to know its Fully Qualified Domain Name or IP. Do you remember the last time you had to set up email on your smartphone? Chances are, you needed to know the FQDN of the email server (if it wasn’t predetermined for you).

In conclusion

If you didn’t know what a Fully Qualified Domain Name was before, you’re probably amazed by how frequently you’ve been interacting with FQDNs in your day-to-day online activities and the great Domain Name System running in the background (or is it just me?). So go give it a try — look up your computer’s FQDN or impress your friends with your knowledge.