Hi Wilcats,
It's got to do with network classes. Classes and subnets go hand in hand:
Imagine an IP address like aaa.bbb.ccc.ddd
If you look at the first number in the IP adress (aaa), then it looks like this:
aaa is 1-126
Class A, network address is aaa (1-126), host address is bbb.ccc.ddd, subnet is 255.0.0.0, max 255^3 PCs per network.
aaa is 128-192
Class B, network address is aaa.bbb, host address is ccc.ddd, subnet is 255.255.0.0, max 255^2 PCs per network.
aaa is 192-223
Class C, network address is aaa.bbb.ccc, host address is ddd, subnet is 255.255.255.0, max 255 PCs per network.
aaa is 0, 127, 224-255
These addresses is reserved for special purposes <=> don't use.
Hope this can guide you in the right direction!
Good luck
Jakob