A nice tool to experiment with subnets. Link to Subnet Calculater
The Subnet calculator start of with mask /25. If you are using 25 bits, your subnet mask is (check the lower part of the tool) 255.255.255.128, you can divide a old Class C network into two parts. Now slide the network address slider past 127, you will see that the Network Address changed from 192.168.0.0 to 192.168.0.128. You can also see that the broadcast address changed. If you want to make a list of all possible CIDR networks from the chosen mask/address press the Generate subnet list.
Hopefully this will give you an idea of how sub netting works