IPv4 classes (legacy first-octet patterns): What is the first-octet bit pattern that identifies a Class B network address?
-
A01xxxxxx
-
B0xxxxxxx
-
C10xxxxxx
-
D110xxxxx
-
E1110xxxx
Answer
Correct Answer: 10xxxxxx
Explanation
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:
Recall classful leading bits: A = 0, B = 10, C = 110.Apply to the first octet of Class B: 10xxxxxx.Confirm decimal range: 128–191 inclusive.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