Microsoft Online Services: tenants, subscriptions and domain names

I often come across confusion with clients trying to understand the differences between tenants, subscriptions and domain names when deploying Microsoft services. This post attempts to clear up some misunderstandings and to – hopefully – make things a little clearer.

Each organisation has a Microsoft Online Services tenant which has a unique DNS name in the format organisationname.onmicrosoft.com. This is unique to the tenant and cannot be changed. Of course, a company can establish multiple organisations, each with its own tenant but these will always be independent of one another and need to be managed separately.

It’s important to remember that each tenant has a single Azure Active Directory (Azure AD). There is a 1:1 relationship between the Azure AD and the tenant. The Azure AD directory uses a unique tenant ID, represented in GUID format. Azure AD can be synchronised with an existing on premises Active Directory Domain Services (AD DS) directory using the Azure AD Connect software.

Multiple service offerings (services) can be deployed into the tenant: Office 365; Intune; Dynamics 365; Azure. Some of these services support multiple subscriptions that may be deployed for several reasons, including separation of administrative control. Quoting from the Microsoft documentation:

“An Azure subscription has a trust relationship with Azure Active Directory (Azure AD). A subscription trusts Azure AD to authenticate users, services, and devices.

Multiple subscriptions can trust the same Azure AD directory. Each subscription can only trust a single directory.”

Associate or add an Azure subscription to your Azure Active Directory tenant

Multiple custom (DNS) domain names can be applied to services – so mycompany.com, mycompany.co.uk and myoldcompanyname.com could all be directed to the same services – but there is still a limit of one tenant name per tenant.

Further reading

Subscriptions, licenses, accounts, and tenants for Microsoft’s cloud offerings.

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