Subnetting Assistance

Hello everyone,

I'm on subnetting.net trying to figure out subnetting through the amount of host and subnets required. I am mostly getting wrong answers by just a bit or two and I'm not seeing what I'm doing wrong. Some help would be greatly appreciated.

Example 1-

"What subnet mask would you use for the 172.0.0 network, such that you can get 170 subnets and 170 host per subnet?"

My work, 2048 1024 512 256 | 128 64 32 16 8 4 2 1

255.255. (class B)

Borrow from 256, so that's 9 bits. So convert 8 of those to 255 and the last one to 128 right?

So 255.255.255.128... Which isn't the right answer. Apparently the answer is 255.255.255.0.


Example 2-

"What subnet mask would you use for the 172.26.0.0 network, such that you can get 1670 subnets and 30 hosts per subnet?"

Class B again. So 255.255 from the start.

2048 1024 512 256 | 128 64 32 16 8 4 2 1

Borrow from 2048, That's 12 bits. Convert 8 of those to 255 and the next 4 from 128-16. Which is 240.

So 255.255.255.240. But it says the answer is 255.255.255.224.

any assistance on where I'm going wrong would be appreciated.

Thanks.