172.16.85.0/21 IP Address Range

A network administrator has been given a network of 172.16.85.0/21 and wants to know the usable range of IP addresses on that subnet.

Which of the following indicates the correct IP address range?

A.

172.16.80.0-172.16.85.254

B.

172.16.80.0-172.16.88.255

C.

172.16.80.1-172.16.87.254

D.

172.16.80.1-172.16.85.255

C.

The network address 172.16.85.0/21 has a subnet mask of 255.255.248.0, which means the network has 11 bits for host addresses, resulting in 2^11 - 2 = 2046 possible host addresses.

The formula to calculate the usable IP address range for a given subnet is:

Network address + 1 (for the first usable host address) to Broadcast address - 1 (for the last usable host address)

To calculate the network address, we need to convert the subnet mask to binary and perform a bitwise AND operation with the given IP address.

Subnet mask: 11111111.11111111.11111000.00000000 IP address: 10101100.00010000.01010101.00000000 &---------------------------------- Network address: 10101100.00010000.01010000.00000000 (172.16.80.0)

The broadcast address is the last address in the subnet, which can be calculated by setting all the host bits to 1:

11111111.11111111.11111000.00000000 (subnet mask)

11111111.11111111.11111111.11111111 (binary 1's)

Converting the binary 1's to decimal gives us 255.255.255.255, which is the broadcast address.

Therefore, the usable IP address range for the subnet 172.16.85.0/21 is:

172.16.80.1 - 172.16.87.254

Option C, 172.16.80.1-172.16.87.254, is the correct answer.