Subnetting is to act as a network address of the subnet IP address by borrowing in a number of bit host address, so that the original network is divided into several subnets.
When subnetting, the subnet address as borrowing host bits increases, the number of subnets increases, but each subnet number of hosts available gradually reduced
The Class C addresses, the original 8-bit host, i.e., 2 ^ 8-2 254 host addresses, subnet mask, default 255.255.255.0. (All-1 or all-0 is not available)
borrow a host bits, to generate 2 ^ 1-2 = 0 subnets, each with a host address 2 ^ 7;
borrow 2 host bits, to generate 2 ^ 2-2 = two subnets, each have 2 ^ 6 host address;
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