Why can’t I use 255.255.255.128 ??

Hello, everyone!

Here’s a simple question, I hope!

I need to create two subnets. Everything I understand suggests I’d mask it using 255.255.255.128, but everyone I talk to tells me no. Isn’t this right:

255.255.255.0 (one subnet: 1-254)
255.255.255.128 (two subnets: 1-127, 129-254)
255.255.255.192 (four subnets: 1-63, 65-127, etc..)