Every AD DS forest has at least one domain. When you set up your first domain forest, it is also set up as a default domain. There are a few reasons why you will need to consider having multiple domains in a forest:
- Smaller administrative boundaries: Active Directory is capable of managing nearly 2 billion objects. Having a large directory creates administrative nightmares. Imagine managing a large herd of sheep. As the herd grows, shepherds need to put in more and more effort to manage it. Predators will also take advantage of it, and, sometimes, shepherds may not notice missing sheep as they are too busy managing the herd. Instead of managing a large number of sheep together, isn't it easy if each shepherd manages smaller herds? Domains will help set smaller administrative boundaries and smaller management targets. This will help manage organization...