Why is the internal network not working in Virtual Box?

Question

virtual box

In virtual box I have 2 VMs on windows 7 pro.
I connected both machines through the VM properties to the internal intnet network.
By means of TFTP DHCP is lifted by one machine. Other windows does not receive addresses.
If you register ip-addresses manually, then the machines still do not see each other and pings do not pass.
If you bring the machines to the network bridge, then the machines see my router and have the Internet. But for tests, isolation from external networks is necessary.
Where to look for an error?
Virtual Box latest version and ExtPack installed on the VM.