Parent-Child Domains

Hi,

Ive created a parent domain named abc.com (192.168.0.1/24) and a child domain xyz.abc.com (192.168.1.1/24) by using the DNS of the parent domain. All them pass to a router w/c link them together. The problem is when im in the parent domain (abc.com) Active Directory and trying to connect to my child domain. it cannot be contacted to. while when im in the child domain (xyz.abc.com) AD and trying to connect to parent domain everything seems fine. I want to know why is this?