Which CIDR block provides 6 usable hosts?

• A. /27
• B. /28
• C. /30
• D. /29

Answer: (D)

The essential idea is counting usable hosts by looking at how many total addresses a subnet provides and then excluding the network and broadcast addresses. In IPv4, a subnet with a prefix length N has 2^(32−N) total addresses, and the number of usable hosts is 2^(32−N) − 2 (except for special cases like /31 or /32).

For a 29-bit prefix, there are 32−29 = 3 host bits, so total addresses = 2^3 = 8. Subtract the network and broadcast addresses, and you’re left with 6 usable hosts. The other prefixes yield different numbers of usable hosts: a 27-bit prefix gives 32 total addresses (30 usable), a 28-bit prefix gives 16 total addresses (14 usable), and a 30-bit prefix gives 4 total addresses (2 usable). Therefore, the 29-bit prefix is the one that provides exactly six usable hosts.