35 years ago the IETF defined special IPv4 addresses in RFC 1597 to be used solely in private intranets and with that created the separation between internal networks and the internet. 5 years later they proceeded to define special-use top-level domains like
.test in RFC 2606. However, the IETF did not reserve a top-level domain to be used within the private intranets that they had introduced before. This lead to confusion with administrators that still persists to the current day.
Every time an administrator was tasked to design a network structure they also had to think about naming conventions within the network. As there was a pre-defined set of "valid" top-level domains within the internet, it was rather easy to just select an "invalid" TLD and use that for the private network. At least, until the ICANN decided to delegate new TLDs to anyone who was willing to apply and pay the required fees. The ICANN even provided a paper on how to identify and mitigate name collisions to be used by professionals.
With RFC 6762 the
.local TLD was approved as a special-use TLD to be used in internal networks and many administrators thought that this would solve the problems, but unfortunately, it did not.
.local was designated to be used with multicast DNS, meaning that each device in a network could grab its preferred hostname. At the beginning this was not a big deal, but as more and more devices implemented Apple's Bonjour protocol (also known as zeroconf) interoperability problems started to pop up. Appendix G of said RFC even mentioned alternative TLDs that might be used, however, the ICANN did not accept any of them as special-use domain names.
Years later the new RFC 7788 defined
.home as a potential local-only TLD, but it was changed to the rather unusual
.home.arpa domain name with RFC 8375. This domain is safe to be used in local networks and was accepted as a special-use domain name by the ICANN. Unfortunately, due the word "home" in the domain it is not quite fitting for business environments.
There are two more TLDs that might be safe to use:
.home are not officially recognized special-use domain names but the ICANN has refrained from delegating them to bidders as the risk of breaking internal networks is deemed to be too big.
As of today, most tutorials still propose to use a publicly registered domain for internal networks. This is why I registered
.localzone.xyz about half a year ago. It is explicitly meant to be used locally, has public DNS records set to prevent any public CA from issuing trusted TLS certificates for that domain as well as DNS records defining all mails using that domain as SPAM and will not be used for publicly accessible services as long as it is owned by me. I am using this domain internally myself and am also trying to get the domain added to the public suffix list. This list defines domain and cookie boundaries so that e.g.
example-a.com is not able to set cookies for
So if you are looking for a domain that you can use for internal domain names then look no further: Either use the special-use domain name
.home.arpa (which might not be fitting for businesses) or use
.localzone.xyz as I am. It makes clear that you are in a local environment without being too specific about whether it is a private or business network.