IPv4 classes (legacy first-octet patterns): What is the first-octet bit pattern that identifies a Class B network address?

Introduction / Context:Historically, IPv4 addresses were divided into classes based on the leading bits of the first octet. While CIDR replaced classful routing, the leading-bit patterns still appear on certifications and in legacy documentation.

Given Data / Assumptions:

• Class A addresses start with binary 0.
• Class B addresses start with binary 10.
• Class C addresses start with binary 110.
• The question asks for the Class B pattern in the first octet.

Concept / Approach:Interpret “x” as any bit (0 or 1). The first two bits of a Class B address are 10, leaving six bits unspecified in the first octet. Therefore, the generic pattern is 10xxxxxx and corresponds to decimal 128–191 in the first octet.

Step-by-Step Solution:

Verification / Alternative check:Compare with Class C pattern 110xxxxx (192–223) and Class A 0xxxxxxx (0–127) to ensure no overlap.

Why Other Options Are Wrong:

• 01xxxxxx: not a classful prefix; first bit must be 1 then 0 for Class B.
• 0xxxxxxx: Class A pattern.
• 110xxxxx: Class C pattern.
• 1110xxxx: Class D (multicast) leading bits, not unicast class B.

Common Pitfalls:Confusing Class C (110) with B (10); equating decimal boundaries with arbitrary prefixes.

Final Answer:10xxxxxx