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I've created 4 virtual machines 2 windows 2008 r2, 1 windows 7 Pro, 1 windows XP Pro SP2. One of the 2008r2 has been promo to (root)DC and both win7 and win XP are able to join to the domain. All guest machines are set 2 network adapters, 1 for NAT and another for Bridge. Here is the TCP/IP configuration of all:

Winows 2008r2 (root DC)
IP address: 10.0.3.1
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

Win 7 Pro
IP: 10.0.3.62
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

Win XP Pro
IP: 10.0.3.61
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

2nd Windows 2008r2 (Fresh installed without any role)
IP: 10.0.3.2
Mask: 255.255.255.0
Gateway: 10.0.3.1
DNS: 10.0.3.1

Problem/Question:
No matter tried to dcpromo the 2nd 2008r2 to another domain controller or as a child domain, there is always comes out the ERROR_TIMEOUT. Try to ping from both win7 and winXP to 10.0.3.2 and vice verse is success without any problem. However, cannot ping from 10.0.3.2 to root dc(10.0.3.1) and from root dc to 10.0.3.2. I've been tried to disable all the NAT adapter inside the network and only left the "Bridge" having the same issue. Anyone can help me? Thanks a lot.

The easiest and for most the best way to use and test Windows domains in VirtualBox
Setup Windows Domain in VirtualBox

This isolates the domain from the rest of your LAN making it much easier to do. As for the actual configuration issues you would need to ask the MS Windows networking forum as this is outside the scope of the VirtualBox forum.

As Perry suggested, you really need to have the domain running in its own network and its own IP subnet. All domain members should use the DC only for DNS but should not use the DC as a gateway address - the DC is not a router. The gateway address will be the local IP of the router which connects your domain network to the public network.

My setup looks like this.

Internet
|
DSL router
192.168.0.1
|
192.168.0.x dg 192.168.0.1 dns 192.168.0.1 physical network and physical machines (including VirtualBox host)
|
192.168.0.99 dg 192.168.0.1
NAT router (vm)
192.168.21.254 dg blank
|
192.168.21.21 DC (vm) dns 192.168.21.21 dg 192.168.21.254
|
other domain machines (vms) 192.168.21.x dns 192.168.21.21 dg 192.168.21.254

The DNS in the DC is set to forward to a public DNS service so that it can resolve foreign URLs for both itself and other domain members.