Calculating Usable Host Addresses with a 23-Bit Network Mask

A

To determine how many usable host addresses there will be per subnet, we need to first determine the network and broadcast addresses, and then subtract them from the total number of IP addresses in the subnet.

In this case, the network block 209.18.12.0 will be segmented using a 23-bit network mask. A 23-bit network mask uses 23 bits for the network portion of the address, leaving 9 bits for the host portion of the address. The binary representation of the 23-bit network mask is:

11111111.11111111.11111110.00000000

Converting this binary representation to dotted decimal notation gives us:

255.255.254.0

The network address for the first subnet will be the lowest address in the subnet, which in this case is 209.18.12.0. The broadcast address for the first subnet will be the highest address in the subnet, which is the last address before the next network address. To determine the broadcast address, we set all of the host bits to 1 in the subnet mask:

11111111.11111111.11111110.00000000

Converting this binary representation to dotted decimal notation gives us:

255.255.254.255

Therefore, the usable IP addresses in each subnet will be the total number of IP addresses in the subnet minus 2 (one for the network address and one for the broadcast address):

2^(32-23) - 2 = 2^9 - 2 = 510

Therefore, the answer is A. 510 usable host addresses per subnet.