How to Add Child Domains:
1. Create a DNS deligation (mgt of that particular domain is done by a separate server)(on parent domain)
2. DCPROMO on child domain
Global Catalog Service becomes very important when we add Child Domain. Its a fast search index of all the objects in AD forest. Its a super Db.In large site its recommended to have a GC, but in small site it may be a problem. in small site we can rather use UGMC(Univ Group Member Caching). UGMC refresh automatically, its a compromise solution if we have a small site