A Top-Level Domain (TLD) is the part of the domain name located to the right of the dot (“.”) The TLD is the last segment of the domain name, the letters immediately following the final dot in an Internet address. A TLD identifies something about the associated website, like its origin (country code for example) or the nature of its organization.
The top-level domain “FOO” however, it is worth mentioning, is a hypothetical placeholder and not an actual existing or functioning TLD. It is used in computer science and related fields such as domain name systems and programming as a nonspecific reference in contexts where the actual value is irrelevant or can be replaced with the actual value by the users or testers involved. For example, a technical documentation might instruct the user to “enter your domain name, such as ‘example.foo’”. It’s a convention to exemplify user inputs or variables as seen in this extract from Google “If you’re testing SPF records, you can replace example.com with your domain (for example, yourdomain.foo)”.
This placeholder use of the term “FOO” dates back to the 1930s and the acronym FUBAR from the US military: “F\*\*\*ed Up Beyond All Recognition/Any Repair/All Reason”, as mentioned in The New Hacker’s Dictionary, Third Edition. In computing, the term “foo” is commonly used alongside “bar” or “baz” as a metasyntactic variable – a word used to represent another word, sentence, or data.
As per ICANN rules, real TLDs for the control and management of the domain name system fall under several categories. These include generic TLDs (like .com, .org, .info), country code TLDs (like .us, .ca, .jp), and new ICANN-defined categories including infrastructure top level domain (.arpa) and sponsored top-level domains (.aero, .coop, .museum).
In the latest round of TLD expansion, ICANN introduced the concept of Generic Top-Level Domains (gTLDs), which are TLDs that are open to registrants around the globe, and can be almost any term or string of characters. Businesses and other organizations can apply to create a gTLD that matches their name or brand, such as .google, .amazon etc.
For more precise information on TLDs, you may visit the ICANN (Internet Corporation for Assigned Names and Numbers) website, the body that oversees domain names and TLDs.
Sources:
- Google Administrative help: https://support.google.com/a/answer/2466563?hl=en
- ICANN’s definition of TLD: https://www.icann.org/resources/pages/tlds-2012-02-25-en
- The New Hacker’s Dictionary, Third Edition: https://archive.org/details/newhackersdictio00raym
- Microsoft Azure Documentation: https://docs.microsoft.com/en-us/azure/frontdoor/front-door-url-rewrite.
- RFC3092 (Etymology of “Foo”): https://www.rfc-editor.org/rfc/rfc3092.
