Windows Server 2003 introduced significant improvements to Active Directory, including cross-forest trusts, application partitions, and the ability to rename domains. Designing your AD infrastructure correctly from the outset is critical, as restructuring later involves significant downtime and risk.
Forest and Domain Planning
The forest is the ultimate security boundary in Active Directory. Most organizations should start with a single-forest, single-domain model unless regulatory or political requirements mandate separation. Each additional domain adds management overhead and complexity to authentication flows, so avoid creating domains solely for organizational convenience.
Organizational Units provide the flexibility to delegate administration and apply Group Policy without the overhead of separate domains. Design your OU structure around administrative delegation needs rather than mirroring the company org chart, as organizational changes are far more frequent than changes in administrative responsibility.
Site topology in AD controls replication and client authentication behavior. Define sites to match your physical network topology, create site links with appropriate replication schedules, and place domain controllers in each site where users require local authentication. Properly configured sites ensure that users authenticate against local DCs and reduce WAN traffic for replication.