Networking troubles

I have a tough go about trying to subnet and can't seem to grasp the concept , here is one ouestion
Chapter 5Guide to Networking Essentials (Gregory Tomsho)

15. For a Class C network address 192.168.10.0 , which of the following subnet masks provides 32 subnets ??
b.255.255.255.248
but how ? ggrrrrr
any help would be appreciated :-)Kris

Each time you borrow an additional bit in the subnet mask you cut the available range in half. A /24 subnet borrows 0 bits, therefor has a range of 256.

Borrow 1 bit and it's TWO ranges of 128
Borrow 2 bits and it's FOUR ranges of 64
Borrow 3 bits and it's EIGHT ranges of 32
...

Therefore your total address space is always the same. Take a pizza for example. If you make zero cuts you have a whole pizza. 1 cut and you have two halves. 2 cuts and you have Four quarters, 4 cuts (because you have to cut each half in half) you have 8 1/8ths and so on.

192.168.10.0
255.255.255.248

248 in Binary 11111000 = 2*5 = 32 Subnets( 5 bits on)
2*3 - 2 = 6 Hosts per subnet (3 bits off)

subtract 2 for subnet address and broadcast address.

Valid subnets 256 - 248 = 8
Hence 0,8,16,24................248

0 to 255 >>> 256

Source : CCNA SYBEX TODD LAMMLE