Question

What is the subnet id of a host with an IP address 172.16.66.0/21?

(a) 172.16.36.0

(b) 172.16.48.0

(c) 172.16.64.0

(d) 172.16.0.0

Answer 1

Right answer is (c) 172.16.64.0

Easiest explanation: A /21 is 255.255.248.0, which means we have a block size of 8 in the third octet, so we just count by 8 until we reach 66. The subnet in this question is 64.0. The next subnet is 72.0, so the broadcast address of the 64 subnet is 71.255.