IPv4 subnetting design: For network 172.16.0.0, you need 25 subnets for your company now and an additional 30 after a merger (total 55). Each subnet must support about 600 hosts. What single subnet mask will satisfy these requirements?

Introduction / Context: Subnet mask selection must satisfy two constraints simultaneously: number of required subnets and number of hosts per subnet. For a Class B private block (172.16.0.0/16), borrowing bits from the host field increases subnet count while reducing hosts per subnet. The chosen mask must meet or exceed both the subnet and host requirements.

Given Data / Assumptions:

• Base network: 172.16.0.0 with default mask /16.
• Required subnets: 25 now + 30 later = 55 total.
• Hosts per subnet ≈ 600 (usable addresses need to exceed this).
• Standard IPv4 addressing with CIDR.

Concept / Approach: Let s = subnet bits borrowed from the /16 host field. Subnets available = 2^s. Hosts per subnet (usable) = 2^(16 - s) - 2. We need 2^s ≥ 55 and 2^(16 - s) - 2 ≥ 600. Find the smallest s that satisfies both.

Step-by-Step Solution:

Verification / Alternative check: Check neighboring masks: /21 (255.255.248.0) gives 2^5 = 32 subnets (too few). /23 (255.255.254.0) would give more hosts than needed but still only 2^7 = 128 subnets if taken from /16? (That would be /23 = s = 7, but then hosts per subnet = 510, which fails the ~600 requirement.) /22 is the smallest mask that simultaneously satisfies both bounds.

Why Other Options Are Wrong:

Common Pitfalls: Forgetting to satisfy both constraints, miscounting usable hosts by ignoring the network and broadcast addresses, and mixing classful intuition with CIDR math.

Final Answer: 255.255.252.0.