AD subnet definition in hosted environment

I am wondering what is the best practice when defining subnets for machines in a commercially hosted environment (in our case Rackspace).

Each time a new machine is created it gets a widely differing ip number from "my" other machines on their networks. It is not like adding a new server to an existing office infra.

I have an AD site for those servers but which is better:

a.) make a few subnets that cover every one of the thousands and thousands of possible Rackspace VM's or

b.) each time get the ip of the particular server and add a new subnet to AD with a 32 bit mask (are 32 bit masks even ok in AD subnets)?

Regards, James.

James.