Subnetting a Class B Network - Cisco CCNA Exam Question

How to Subnet a Class B Network for 100 Subnetworks with 500 Host Addresses per Subnet

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Question

A network administrator is planning a network installation for a large organization. The design requires 100 separate subnetworks, so the company has acquired a

Class B network address. What subnet mask will provide the 100 subnetworks required, if 500 usable host addresses are required per subnet?

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Explanations

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A. B. C. D. E. F.

D

To create 100 subnetworks with 500 usable host addresses per subnet, we need to find the subnet mask that can accommodate this requirement. We can use the formula 2^n, where n is the number of bits used for the network portion of the address, to calculate the number of subnets that can be created with a given subnet mask. Similarly, we can use the formula 2^h - 2, where h is the number of bits used for the host portion of the address, to calculate the number of usable host addresses per subnet.

Since the organization has acquired a Class B network address, the default subnet mask is 255.255.0.0 or /16, which provides 1 network and 65,534 host addresses. However, this default subnet mask does not allow for the creation of 100 subnetworks with 500 usable host addresses per subnet.

To accommodate 100 subnetworks, we need to use at least 7 bits for the network portion of the address. The reason we need 7 bits is because 2^7 = 128, which is greater than 100. Therefore, we can use a subnet mask of /23, which provides 7 bits for the network portion of the address and 9 bits for the host portion of the address.

To accommodate 500 usable host addresses per subnet, we need to use at least 9 bits for the host portion of the address. The reason we need 9 bits is because 2^9 - 2 = 510, which is greater than 500. Therefore, we can use a subnet mask of /23 + 9 = /32 - 9 = /23.9 or /27, which provides 9 bits for the host portion of the address.

The subnet mask that meets the requirement of 100 subnetworks with 500 usable host addresses per subnet is 255.255.255.128 or /25, which provides 128 subnets and 126 usable host addresses per subnet.

However, none of the given answers match the correct subnet mask. The closest answer is B. 255.255.248.0 or /21, which provides 8 bits for the network portion of the address and 11 bits for the host portion of the address. This subnet mask can accommodate 512 subnets and 2,046 usable host addresses per subnet, which is more than the required 100 subnetworks with 500 usable host addresses per subnet.

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