Subnet Mask for Subnetting Eight LANs with 5-26 Hosts

# Appropriate Subnet Mask for Subnetting Eight LANs

### Question

Which subnet mask would be appropriate for a network address range to be subnetted for up to eight LANs, with each LAN containing 5 to 26 hosts?

### Explanations

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A. B. C. D. E.

D

We need 26 usable IP addresses, so we need subnets with a range of 32 addresses. This is the /27 netmask, or 255.255.255.224.

To determine the appropriate subnet mask for a network address range to be subnetted, we need to know the number of LANs and the number of hosts in each LAN.

In this scenario, we have up to eight LANs, with each LAN containing 5 to 26 hosts. We can use the formula 2^n - 2 to calculate the number of hosts in each subnet. The -2 is used because the first and last IP addresses in each subnet are reserved as the network and broadcast addresses, respectively.

For the smallest LAN with 5 hosts, we need to reserve 3 bits for the host portion of the IP address (2^3 = 8, minus 2 reserved addresses). For the largest LAN with 26 hosts, we need to reserve 5 bits (2^5 = 32, minus 2 reserved addresses).

To accommodate all the LANs, we need to reserve at least 5 bits (2^5 = 32, which is greater than 26). This means that we need to use a subnet mask with at least 8 bits for the network portion of the IP address (32 bits - 5 bits = 27 bits). The subnet mask that has 8 bits for the network portion is 255.255.255.0 (11111111.11111111.11111111.00000000 in binary).

However, we also need to ensure that we have enough subnets to accommodate up to 8 LANs. To do this, we need to reserve enough bits for the subnet portion of the IP address. In this case, we need to reserve 3 bits for the subnet portion (2^3 = 8 subnets). This means that we need to use a subnet mask with at least 27 bits for the network and subnet portions of the IP address (32 bits - 3 bits = 29 bits). The subnet mask that has 27 bits for the network and subnet portions is 255.255.255.224 (11111111.11111111.11111111.11100000 in binary).

Therefore, the appropriate subnet mask for this network address range to be subnetted for up to eight LANs, with each LAN containing 5 to 26 hosts, is D. 255.255.255.224.