I'm using AWS and have a VPC which is connected to the local network via a VPN. I can launch EC2 instances, put them in various subnets in various AZ's and connect to them via internal IP.
I can also join them to the local domain.
However, I want to be able to automatically put them into specific groups within the AD domain, based on their IP address, when they join the domain.
Therefore, when I create a new instance of my image it will be joined to the domain and because it has a particular IP address, will be added to a certain group.
I don't think IP's are the right thing to key off of here. Are you automatically launching your instances into the VPC? If so, you could tag them or give them a 'join_X_group' role that would let them talk to to your DC in that region. Even having a DHCP Options Set that would have them come up with a specific DNS server which could be your DC and then follow http://docs.aws.amazon.com/workspaces/latest/adminguide/join_a_directory.html#join_instance to join the host to a domain with a user-data script.
Alternatively, you could create a Directory (see the Workspaces panel) that replicates from your existing AD infrastructure, and join it that way.