Discussion
Which one of the following CIDR prefixes exactly represents the range of IP addresses 10.12.2.0 to 10.12.3.255?
Answer: A
The prefix is 10.12.2.0/23.
The range 10.12.2.0 to 10.12.3.255 holds \(2 \times 256 = 512 = 2^{9}\) addresses, so 9 host bits and a prefix length of \(32 - 9 = 23\). A /23 block must start on an even third octet; 2 is even (binary 0000 0010), so 10.12.2.0/23 spans exactly 10.12.2.0 to 10.12.3.255.
10.12.2.0/24 covers only 10.12.2.0 to 10.12.2.255. 10.12.0.0/22 covers 10.12.0.0 to 10.12.3.255, twice the range. 10.12.2.0/22 is not a valid block start, since a /22 must begin at a third octet divisible by 4; it denotes the same 10.12.0.0 to 10.12.3.255 block.
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