There is also a free IP subnet calculator that you can download from the Boson web site. It can help you understand how it all works.
Hope this helps,
The no. of networks depends not only on subnet mask but also with the network id u have.
For exampls if u have class A 10.0.0.0 255.255248.0
it will give u 8192 subnets and 2048 host per subnet.
if u have class B 184.108.40.206 255.255.248.0
the same subnet mask will yield u only 32 subnets of 2048 hosts.
u can make u of subnet calculators to calculate no. of nodes and subnets
Hope this clarifies ur first doubt.
What u mean by even numbers ?????