I'm running into this issue on a Hyper-V Server running on WinSrv 2008 R2
I have downloaded Exchange Server 2010 SP1 trial Virtual Machine. The trial has Exchange server roles installed and configured and is set up as a DC for the Contoso domain. The VM is set up with a static IP address and static DNSserver address, rather than dynamic. In this configuration, it has no connectivity to the Internet and I can’t add any other VMs to the domain. I'd used the directions in the Quick Start Guide to create and use an External network type to connect to
the Internet, but that did not work.
To fix the access problem, I ran the Networking troubleshooter and it found the ‘error’ that DHCP was not configured for the local area connection. I had the troubleshooter automatically fix it, which it did by setting up dynamic IP address and obtaining
the DNS server address automatically. The enabled Internet access and allowed me to add the other VM to the domain, but it screwed up all of the Exchange server settings on the server. I couldn’t use any of the management tools and it took down all of the
defaults Web sites that Exchange uses.
I ended up rolling back to the original state of the Exchange VM, so now Exchange works, but I still have to get Internet access and re-add the another VM to the domain, which I haven't been able to do.
I created an External Virtual network and bound it to my physical NIC, as described in the setup guide provided by the Exchange team, but that has not worked either.
By the way, if you want to let the virtual machine access internet, the easiest way should be create an External Virtual Network bind to one physical network adapter and then attach the network adapter of the virtual machine to this External Virtual Network.
If you have a router or DHCP server in your LAN, the virtual machines connect to the External Virtual Network will get IP addresses from them.