In a class A, B, or C network, two IP addresses are unavailable without dividing the subnet: network number and broadcast address. For example, in a class C network without a subnet, 202.203.34.0 is used to represent the network number, and 202.203.34.255 is used to represent the broadcast address. Because there are 256 IP addresses in the class C network, subtract the two IP addresses, then there are only 256-2 = 254 available IP addresses. If you divide a class C network into four subnets, how many unavailable IP addresses will be added? You can think like this: when the class C network is not divided into subnets, there are two IP addresses unavailable; now the class C network is divided into four subnets, therefore, two IP addresses are unavailable in each subnet. Therefore, eight IP addresses are unavailable in the four subnetworks. Eight IP addresses are used to remove the two unavailable IP addresses when no subnet is divided, the result is 6. Therefore, after dividing a class C network into four subnets, there will be six more unavailable IP addresses.
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