IPv4 classes and defaults: given the IP address 201.142.23.12 (a Class C address), what is the default subnet mask that applies before any custom subnetting?

Introduction / Context: In classical IPv4 addressing (pre–Classless Inter-Domain Routing), each address fell into a class with a default subnet mask. Recognizing the default mask for a given dotted-decimal address helps with quick network/broadcast calculations and legacy exam questions. Here, we analyze address 201.142.23.12 and identify its default mask prior to any subnetting changes.

Given Data / Assumptions:

• Address: 201.142.23.12
• We assume traditional classful defaults (no VLSM/CIDR overrides).
• We seek the default mask only, not a designed subnet mask.

Concept / Approach: Classful ranges by first octet are: Class A = 1–126 → default mask 255.0.0.0; Class B = 128–191 → default mask 255.255.0.0; Class C = 192–223 → default mask 255.255.255.0. Since 201 falls within 192–223, 201.x.y.z is Class C, so the default mask is 255.255.255.0.

Step-by-Step Solution:

Verification / Alternative check: Convert 201 to binary (11001001). The leading bits for Class C are 110, which matches the 192–223 range and therefore the /24 (255.255.255.0) default network mask in classful notation.

Why Other Options Are Wrong:

Common Pitfalls: Confusing classful defaults with designed subnet masks; mixing up Class B and C boundaries (191 vs 192). Remember, 201 clearly lies in Class C.

Final Answer: 255.255.255.0.