There are some domain extensions you will be unable to register. Restricted TLDs exist because they are intended for limited use or needed for testing or documentation purposes.
Reserved DNS Names
The TLDs .invalid, .example, .localhost and .test, were reserved by the Internet Engineering Task Force (IETF). They can’t be installed into the root zone of the DNS to reduce the chance of conflict or confusion. These names can then be used for either local testing scenarios for documentation purposes.
• Invalid – used to demonstrate domains that are invalid such as those with apostrophes or spaces.
• Example – used in documentation or as examples when presenting the concepts of the DNS system or the Internet in general.
• Localhost – used when the domain refers back to a local computer.
• Test – used for testing code related to DNS.
Restricted Domains
Not all domain extensions are available to the public. There are many restricted top-level domains known as rTLDs.
• To register a domain with an rTLDs, you need to belong to a particular community or represent a specific entity. For example, .name is reserved for individuals and .edu is restricted to educational entities.
• To date, .gov and .mil are controlled by the government of the US, restricted to government and military purposes. The .edu extension is also restricted and only accredited institutes or US-based post-secondary institutions can use it.
• Trademarked names aren’t available to anyone except the trademark owner, for example .versace and .nescafe aren’t open for general registration.