[AzureADDS] Add prefix naming info

Author: iainfoulds

articles/active-directory-domain-services/tutorial-create-instance.md

@@ -63,6 +63,15 @@ When you create an Azure AD DS instance, you specify a DNS name. There are some
 * **Custom domain names:** The most common approach is to specify a custom domain name, typically one that you already own and is routable. When you use a routable, custom domain, traffic can correctly flow as needed to support your applications.
 * **Non-routable domain suffixes:** We generally recommend that you avoid a non-routable domain name suffix, such as *contoso.local*. The *.local* suffix isn't routable and can cause issues with DNS resolution.
 
+> [!TIP]
+> If you create a custom domain name, take care with existing DNS namespaces. It's recommended to include a unique prefix for the domain name. For example, if your DNS root name is *contoso.com*, create an Azure AD DS managed domain with the custom domain name of *corp.contoso.com* or *ds.contoso.com*. In a hybrid environment with an on-premises AD DS environment, these prefixes may already be in use. Use a unique prefix for Azure AD DS.
+>
+> You can use the root DNS name for your Azure AD DS managed domain, but you may need to create some additional DNS records for other services in your environment. For example, if you run a webserver that hosts a site using the root DNS name, there can be naming conflicts that require additional DNS entries.
+>
+> In these tutorials and how-to articles, the custom domain of *contoso.com* is used as a short example. In all commands, specify your own domain name, which may include a unique prefix.
+>
+> For more information, see [Select a naming prefix for the domain][naming-prefix].
+
 The following DNS name restrictions also apply:
 
 * **Domain prefix restrictions:** You can't create a managed domain with a prefix longer than 15 characters. The prefix of your specified domain name (such as *contoso* in the *contoso.com* domain name) must contain 15 or fewer characters.
@@ -226,3 +235,6 @@ To see this managed domain in action, create and join a virtual machine to the d
 [on-prem-sync]: tutorial-configure-password-hash-sync.md
 [configure-sspr]: ../active-directory/authentication/quickstart-sspr.md
 [password-hash-sync-process]: ../active-directory/hybrid/how-to-connect-password-hash-synchronization.md#password-hash-sync-process-for-azure-ad-domain-services
+
+<!-- EXTERNAL LINKS -->
+[naming-prefix]: /windows-server/identity/ad-ds/plan/selecting-the-forest-root-domain#selecting-a-prefix