What's the fastest way to solve the "what subnet Mask to use" subnetting problems?

SurferdudeHB

What subnet mask would you use for the 172.27.0.0 network, such that you can get 40 subnets and 970 hosts per subnet?

Answer: 255.255.252.0

I can't figure this one one.. How do you get the 255.255.252.0 mask?

davenull

Know your powers of 2. For 970 hosts the next power of 2 is 1024 (2**10), so 32 - 10 equals 22. So the mask is going to be /22. To convert that to the old notation I subtract it from the next octet 'boundary' number, 24 in this case, then raise the result to the power of 2 and subtract it from 256.