On a Class C IPv4 network using a subnet mask whose last octet is 192 (/26), how many subnets are available under the legacy convention that disallowed subnet-zero and all-ones subnets?

Introduction:
Subnetting questions often hinge on whether you assume modern CIDR practices (which allow using all subnets, including subnet-zero) or legacy rules (which historically reserved the all-zeros and all-ones subnets). This problem asks specifically about a Class C network with a mask value of 192 in the last octet, i.e., a /26, and wants the number of subnets available under the older, more restrictive convention.

Given Data / Assumptions:

• Class C base: /24.
• Subnet mask: 255.255.255.192 → borrows 2 bits (since 192 = 11000000).
• Legacy rule: do not use subnet-zero or the all-ones subnet.

Concept / Approach:
With 2 borrowed bits, the total number of theoretical subnets is 2^2 = 4. Under the legacy “no subnet-zero” rule, you must subtract 2 reserved subnets (all-zeros and all-ones), yielding 4 − 2 = 2 usable subnets. Note that hosts per subnet would be 2^6 − 2 = 62 (since 6 host bits remain), but the question is about subnets available, not hosts per subnet.

Step-by-Step Solution:
1) Determine borrowed bits: /26 means 2 bits for subnetting.2) Compute theoretical subnets: 2^2 = 4.3) Apply legacy reservation: 4 − 2 = 2 usable subnets.4) Confirm distinction from hosts per subnet (62) to avoid misinterpretation.

Verification / Alternative check:
Modern practice (allowing all subnets) would yield 4 usable subnets. However, many classic exam items explicitly invoke the old restriction to test historical knowledge.

Why Other Options Are Wrong:

• 254 / 62 / 30 / 14: these are typical host-per-subnet counts for /24, /26, /27, /28 respectively, not the number of subnets under the stated convention.

Common Pitfalls:
Confusing “subnets” with “hosts per subnet,” or forgetting whether subnet-zero is permitted in the scenario.

Final Answer:
2