In classful IPv4 addressing, what is the default subnet mask for a Class A network block?

Introduction: Classful IPv4 addressing once defined fixed mask lengths for Classes A, B, and C. Although CIDR superseded classful addressing, exams still test default masks. The question asks for the default mask for a Class A network.

Given Data / Assumptions:

• Class A first octet range (usable): 1–126 (127 is loopback; 0 reserved historically).
• Default mask length: /8 for Class A.
• We distinguish masks from specific addresses like loopback.

Concept / Approach: A Class A network reserves 8 bits for the network portion and 24 bits for hosts by default. In dotted-decimal, /8 corresponds to 255.0.0.0.

Step-by-Step Solution:

Verification / Alternative check: Class B defaults to /16 (255.255.0.0) and Class C to /24 (255.255.255.0), reinforcing that Class A is /8 (255.0.0.0).

Why Other Options Are Wrong:

• 127.0.0.1: loopback address, not a mask.
• 255.255.255.0.0: invalid, five octets.
• 255.255.255.0: /24, Class C default.
• None of the above: incorrect because 255.0.0.0 is correct.

Common Pitfalls: Confusing default masks with commonly deployed masks after subnetting; treating loopback as part of Class A usable range.

Final Answer: 255.0.0.0.