In classful IPv4 addressing, what is the default subnet mask associated with a Class C network?

Introduction / Context:
Classful addressing predates CIDR and assigns default masks to address classes A, B, and C. Many certification and legacy configuration questions still refer to these defaults, so recalling them remains useful even though modern networks use variable-length subnet masks with CIDR notation.

Given Data / Assumptions:

• We are referencing historical classful IPv4 defaults.
• Class C addresses span 192.0.0.0 through 223.255.255.255.
• The question seeks the default mask, not a custom subnet.

Concept / Approach:
Class C allocates 24 bits to the network and 8 bits to hosts by default. The corresponding dotted-decimal mask is 255.255.255.0. Class B and A defaults are 255.255.0.0 and 255.0.0.0 respectively. The host 127.0.0.1 is a loopback address, not a mask, and is included as a distractor.

Step-by-Step Solution:

Verification / Alternative check:
Recall that /24 equals 11111111.11111111.11111111.00000000 in binary, matching 255.255.255.0. Compare with Class B’s /16 (255.255.0.0) and Class A’s /8 (255.0.0.0) to confirm consistency.

Why Other Options Are Wrong:

• 255.255.0.0: This is the Class B default mask.
• 255.0.0.0: This is the Class A default mask.
• 127.0.0.1: Loopback address, not a subnet mask.
• None of the above: Incorrect because 255.255.255.0 is correct.

Common Pitfalls:
Confusing modern CIDR with classful defaults and misidentifying 127.0.0.1 as a mask instead of a host address used for loopback testing.

Final Answer:
255.255.255.0