For a department planning growth within a 10.0.0.0/8 private space, which subnet would be the smallest that can accommodate the finance department’s growth (current 10 hosts, 50% growth)?

• A. 10.0.0.0/8
• B. 10.0.0.0/16
• C. 10.3.0.0/26
• D. 10.3.3.128/27

Answer: (D)

Subnetting is about fitting the required number of hosts into the smallest possible address space. With 10 hosts and 50% growth, you need at least 15 usable addresses. In IPv4, the number of usable hosts in a subnet is 2^(32−n) − 2, where n is the prefix length. A /28 provides 16 total addresses but only 14 usable, which isn’t enough. A /27 provides 32 total addresses and 30 usable, which easily meets and exceeds the 15-host requirement, making it the smallest suitable option. The 10.3.3.128/27 block, in particular, spans 10.3.3.128 through 10.3.3.159, with 10.3.3.128 as the network address and 10.3.3.159 as the broadcast address, leaving 30 usable addresses (10.3.3.129–10.3.3.158). This subnet sits within the 10.0.0.0/8 private space, fulfilling the constraint.