Create a sub net mask for 512 subnets using 172.31.0.0

The default sub net mask for a class b network is 255.255.0.0, working through the borrowing off bits using the formula 2^n, n being the bits borrowed

- 0 bits = 1 Subnet
- 1 bit = 2 subnets
- 2 bits = 4 subnets
- 3 bits = 8 subnets
- 4 bits = 16 subnets
- 5 bits = 32 subnets
- 6 bits = 64 subnets
- 7 bits = 128 subnets
- 8 bits = 256 subnets
- 9 bits = 512 subnets

That gives us a subnet mask off 255.255.255.128 which equates to a subnet prefix off /25

From this how do i find the 256th network?

Many thanks