Which sequence correctly shows the default subnet masks for Class A, Class B, and Class C IPv4 networks?

Introduction:
Legacy classful IPv4 addressing assigns default masks based on address class. Although modern networks use CIDR, knowing the defaults remains useful for exams and interpreting historical configurations.

Given Data / Assumptions:

• Class A ranges start 0–127, default mask /8 → 255.0.0.0.
• Class B ranges start 128–191, default mask /16 → 255.255.0.0.
• Class C ranges start 192–223, default mask /24 → 255.255.255.0.

Concept / Approach:
Match each class to its default prefix length and derive the dotted-decimal mask. The correct order A, B, C corresponds to /8, /16, /24 respectively.

Step-by-Step Solution:
1) Class A → /8 → 255.0.0.0.2) Class B → /16 → 255.255.0.0.3) Class C → /24 → 255.255.255.0.

Verification / Alternative check:
CIDR generalizes these, but routers and textbooks still reference these defaults for historical context and subnetting examples.

Why Other Options Are Wrong:

• Option B: lists the masks in reverse order (C, B, A).
• Option D: includes 255.255.255.255 which is a host route/broadcast context, not a class default mask.
• Option A/E: invalid or unrelated sequences.

Common Pitfalls:
Confusing class defaults with commonly deployed subnetted CIDR masks; the question asks specifically for the default classful masks.

Final Answer:
255.0.0.0, 255.255.0.0, 255.255.255.0