CIDR:
When you've chosen a possible subnet mask for your network and need to determine the number of subnets, valid hosts, and broadcast addresses of a subnet that the mask provides, all you need to do is answer five simple questions:
How many subnets does the chosen subnet mask produce?
How many valid hosts per subnet are available?
What are the valid subnets?
What's the broadcast address of each subnet?
What are the valid hosts in each subnet?
At this point, it's important that you both understand and have memorized your powers of 2.
How many subnets? 2x = number of subnets. x is the number of masked bits, or the 1s. For example, in 11000000, the number of 1s gives us 22. In this example, there are 4 subnets
How many hosts per subnet? 2y- 2 = number of hosts per subnet. y is the number of unmakes bits, or the 0s. For example, in 1100000, the number of 0s gives us 26- 2 hosts. In this example, there are 62 hosts per subnet. you need to subtract 2 fro the subnet address and the broadcast address, which are not valid hosts.
What are the valid subnets? 256 - subnet mask = block size, or increment number. An example would be 256 - 192 = 64. The block size of a 192 mask is always 64. Start counting at zero in blocks of 64 until you reached the subnet mask value and these are your subnets. 0, 64, 128, 192. easy huh?
What's the broadcast address for each subnet? Now here's the really easy part. Since we counted our subnets in the last section as 0, 62, 128, and 192, the broadcast address is always the number right before the next subnet. For example, the 0 subnet has a broadcast address of 63 because the next subnet is 64. The 64 subnet has a broadcast address of 127 because the next subnet is 128. And so on. And remember, the broadcast address of the last subnet is always 255.
What are the valid hosts? Valid hosts are the numbers between the subnets, omitting the all 0s and all 1s. For example, if 64 is the subnet number and 127 is the broadcast address, then 65-126 is the valid host range-it's always the numbers between the subnet address and the broadcast address.
Practice Example for a Class B /23:
172.16.0.0 = Network Address
255.255.255.254.0 or /23 = Subnet
Subnets? 2^7 = 128
Hosts? 2^9 - 2 = 510
Valid Subnets? 256-254-0,2,4,6,8, etc up to 254
Braodcast address for each subnet?
Valid hosts?
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