In classful IPv4, how many distinct network IDs exist in Classes A, B, and C combined (excluding loopback and zero network conventions)?

Introduction: Older, classful IPv4 splits addresses into Classes A, B, and C based on the first octet. A common exam calculation asks for the total number of distinct network IDs available across these three classes, applying standard historical conventions (excluding loopback and the zero network).

Given Data / Assumptions:

• Class A first-octet range: 1–126 (127 is loopback; 0 historically reserved).
• Class B first-octet range: 128–191.
• Class C first-octet range: 192–223.
• We count network IDs, not host counts.

Concept / Approach: Compute the number of network IDs in each class and sum them. For classful addressing, the count is strictly positional based on fixed leading bits in the first octet (A: 0xxxxxxx excluding 0 and 127; B: 10xxxxxx; C: 110xxxxx).

Step-by-Step Solution:

Verification / Alternative check: Binary prefixes (A: 0, B: 10, C: 110) yield the same totals when expanded over the subsequent octets dedicated to network ID within each class.

Why Other Options Are Wrong:

• 2,113,658: off by 4; likely a miscount or an incorrect exclusion.
• 16,384: only the Class B total.
• 126: only the Class A total.
• 2,097,152: only the Class C total.

Common Pitfalls: Confusing host capacity with the number of networks; excluding additional ranges beyond loopback and zero; or including Class D/E, which are not unicast network-ID pools.

Final Answer: 2,113,662 classful network IDs in A + B + C.