You can use some easy math to determine the number of subnets and hosts per subnet for each different mask.
You can see in Table A that the bits that are turned on (1s) are used for subnetting, while the bits that are turned off (0s) are used for addressing of hosts. Cisco does teach a subnet zero assumption but they do not test that way. Subset zero: Take note that in the table below I do not assume subnet zero. Therefore, only the following masks can be used with Class C networks ( Table A). Since the Class C mask only uses the last octet for host addressing, we only have 8 bits at our disposal. To make smaller networks, called subnetworks, we will borrow bits from the host portion of the mask. If we use the default subnet mask with a Class C network address, then we already know that three bytes are used to define the network and only one byte is used to define the hosts on each network. Also, by determining the subnet and broadcast addresses, we can easily determine the host addresses because the valid host range is always the numbers between the subnet address and the broadcast address. Why? Because these are not valid host addresses and cannot be assigned to host configurations.
Once the subnet is determined, the broadcast address must be found.
SEE : Download: Time management tips for tech professionals (TechRepublic) Although IP addressing isn't a network administrator's favorite task, it's a critical skill that you must have. Let's face it, some day you are going to have to subnet a network.