It won't stop spoofing, but if you assume that the botnet hosts are evenly distributed across the address space, you're going to cut the spoofed traffic to 1/(2

It won't stop spoofing, but if you assume that the botnet hosts are evenly distributed across the address space, you're going to cut the spoofed traffic to 1/(256^2). At that point, you might not even notice the DDOS.