The first one is to use an inactive sub-domain of a domain that you use publicly. For instance: ad.company.com or internal.company.com. Advantages to this most-preferred approach includes:

    Only one domain name needs to be registered – even if you later decide to make part of your internal name publicly accessible
    Enables you to simply and separately manage internal and external domains
    All internal domain names will be globally unique

The only microscopic drawback is that you’ll have more to type when entering FQDNs on your internal network, so make your subdomain name as short as possible!

However, if it is not feasible for you to configure your internal domain as a subdomain, you can use another domain that you own, which isn’t used elsewhere. For instance, if your public web presence is company.com, your internal domain can be named company.net, only if it’s registered and if it’s not used anywhere else. The main advantage is that you’ve secured a unique internal domain name. However, the disadvantage is that this approach requires you to manage two separate names.