unable to join a domain form VM

I am having such a difficult time joining a domain from a win7 Virtual machine.
The host machine is the DC and has Ad and DNS installed. It???s Win2k8 r2.
Using hyper-V, I created the guest machine and from that VM, I can access the internet, and I can RDP to the host machine, but I can???t join the domain. Every time, I attempt to do that, I get an error message that says ???unable to join domain. Can???t find (computer name)?????????
Anyone has any idea????

Thanks in advance.