There should only be one prefix you have to worry about and if you forget it you can look at any other computer on the network. Then just assign your servers each a small number.
For your case with VMs coming and going it would not be at all hard (and would probably result in better testing) to go the ISP route and assign a unique name to every address and then just report that name to your testers and devs. Reusing the name is exactly the same as reusing the ip address. Then you just have a series of machine names. testvm1, testvm2, testvm3,
Really none of this is very hard, confusing or cumbersome. It just takes someone asking: "How do I make this work?" instead of thinking "Oh no! that is going to be horrible." and looking for excuses not to make it work.