In the Microsoft ecosystem, Active Directory concepts like “domain” and “forest” primarily exist within on-premises environments or Azure Active Directory (AD) Domain Services, rather than in Azure AD (now called Entra ID), which follows a more cloud-native, tenant-based model. Here’s how they align within the current Microsoft identity platform:
1. Domains and Forests in On-Premises Active Directory
Domain: In a traditional on-premises AD, a domain is a logical grouping of network objects, such as users and computers. It forms the core administrative boundary within AD.
Forest: A forest is a collection of one or more AD domains that share a common schema and configuration. Trust relationships enable cross-domain access within a forest or between multiple forests.
2. Azure AD Domain Services (AD DS)
If you need an Active Directory–like environment in Azure that supports legacy authentication protocols and Group Policy, you can set up Azure AD Domain Services. This service replicates some AD features in the cloud, allowing for a managed domain with features like LDAP, NTLM, and Kerberos.
Forest Structure: When Azure AD Domain Services is enabled, it can operate as a single forest domain. It’s ideal for “lift-and-shift” scenarios where legacy applications still require AD-style authentication but are migrated to Azure.
3. Azure AD (now Entra ID)
Entra ID (formerly Azure AD) doesn’t use the same domain or forest structures. It’s a cloud-based directory built on an identity-as-a-service model, organized by tenants rather than domains or forests.
Tenant Structure: Each Entra ID (Azure AD) instance is a separate tenant, essentially a unique administrative boundary. Within a tenant, you can have multiple custom domains, but these don’t equate to AD domains. They’re more about branding and user identity (e.g., [email protected]), not security or policy boundaries.
No Forest Concept: Since Entra ID is built on a multi-tenant, cloud-native foundation, it does not have a forest model. Instead, tenants rely on Conditional Access Policies, Identity Governance, and multi-tenancy for isolation and control.
Summary of Key Differences
Azure AD (Entra ID) is tenant-based with no forest concept. It’s cloud-native, designed for SaaS applications, SSO, and other cloud scenarios.
Azure AD Domain Services provides a domain-like structure within Azure but is limited to single-forest setups. It’s meant for scenarios where traditional AD services are needed in the cloud.
To manage identity needs fully in a cloud-centric environment, use Entra ID for identity management across Microsoft 365 and other SaaS services, while Azure AD DS can support legacy AD requirements in Azure if needed.
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