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

Introduction / Context: Although modern networks overwhelmingly use classless routing (CIDR), many exam questions still reference legacy classful IPv4 conventions. Under classful rules, each class (A, B, C) had a default subnet mask tied to its first-octet range. This question asks you to recall the default mask for a Class B network.

Given Data / Assumptions:

• Class B first-octet range: 128–191 (with 127 reserved for loopback, not part of Class B).
• Default masks: Class A = 255.0.0.0, Class B = 255.255.0.0, Class C = 255.255.255.0.
• We are not considering VLSM/CIDR; this is strictly the historical default.

Concept / Approach: Class B addresses use the first two octets for the network portion and the last two for hosts. The dotted-decimal mask that expresses “network = two octets, host = two octets” is 255.255.0.0. Options that either shorten the network portion (255.0.0.0) or lengthen it (255.255.255.0) correspond to Classes A and C, respectively, not Class B.

Step-by-Step Solution:

Verification / Alternative check: Any elementary networking reference lists Class B's default as /16 (255.255.0.0). Subnetting exercises frequently start from this base mask before borrowing bits for subnets.

Why Other Options Are Wrong:

• 127.0.01: malformed and also reflects the loopback class (127.x.x.x) rather than a subnet mask.
• 255.0.0.0: Class A default (/8).
• 255.255.255.0: Class C default (/24).
• None of the above: incorrect because 255.255.0.0 is correct.

Common Pitfalls: Mixing up class ranges and masks; forgetting that 127.x.x.x is reserved and not a usable network class for addressing hosts.

Final Answer: 255.255.0.0.