Next, divide 256 by the magic number 64 to find the number of subnets you can expect: 4. You can break out 4 subnets from the Class C 192 value, which is exactly the number of subnets you wanted. Now you can put your infrastructure gear, servers, and printers in the first subnet; your marketing department in the second; your sales department in the third; and IT in the fourth:
Subnet 1, Infrastructure192.16.1.0-63
Subnet 2, Marketing192.16.1.64-127
Subnet 3, Sales192.16.1.128-191
Subnet 4, IT192.16.1.192-255
The numbers in bold type identify the range of addresses available in each subnet. Remember, though, that in each subnet, you can't use the first and last numbers. The four networks in CIDR notation would be 192.16.1.0/26, 192.16.1.64/26, 192.16.1.128/26, and 192.16.1.192/26, respectively.
For Class B subnet values, you must perform one additional calculation to find the number of hosts per subnet. After you find the magic number, you multiply it by 256, then subtract 2. So, for the Class B 192 subnet value, you perform the following calculations:
256 - 192 = 64
64 * 256 = 16,384
16,384 - 2 = 16,382
Thus, the Class B 192 subnet value yields 16,382 usable hosts per subnet. The number of subnets from the Class B subnet 192 value is the same as for the Class C 192 subnet value, which is 4 (i.e., 256 divided by 64).
To find the number of usable hosts for the Class A 192 subnet value, perform the following three calculations:
256 - 192 = 64
64 * 256 * 256 = 4,194,304
4,194,304 - 2 = 4,194,302
The Class A 192 subnet value yields 4,194,302 usable hosts per subnet. The number of subnets is the same as the number of subnets for the Class C and Class B 192 subnet values.
Understanding CIDR notation and magic-number math is the first step to implementing classless subnetting. You'll encounter CIDR notation in support documents, and you'll need the magic-number math when you try to solve subnetting problems. I strongly encourage network administrators considering whether to go through Cisco Certified Network Associate (CCNA) training to do so. You'll receive a wealth of subnetting, switching, and routing information that will prove enormously useful to you in your administration career.