If the host on a network has the address 172.16.66.50/21, what is the subnet ID to which this host belongs?

A) 172.16.48.0
B) 172.16.56.0
C) 172.16.64.0
D) 172.16.72.0

C
Explanation: C) The correct answer is 172.16.64.0 because, first, the resident subnet (the subnet ID of the subnet in which the address resides) must be numerically smaller than the IP address, which rules out one of the answers. The mask converts to 255.255.248.0. As such, you can copy the first two octets of the IP address because of their value, 255. 55. For the third and fourth octet, the subnet ID value must be a multiple of 8 (magic number) because 256 - 248 (mask) = 8. Those multiples include 48, 56, and 64, and the correct choice is the multiple closest to the IP address value in that octet (66.50 ) without going over. So, the correct subnet ID is 172.16.64.0.