In classful IPv4 addressing, what is the valid first-octet range that identifies a Class B network block?

Introduction:
Classful IPv4 addressing historically divided the address space into fixed-sized classes (A, B, C, D, E) based on the first octet. Although classless routing (CIDR) is standard today, many networking exams still ask you to recall the legacy class ranges. This item checks whether you remember the specific first-octet window for Class B networks.

Given Data / Assumptions:

• IPv4 addresses are 32 bits, shown as four decimal octets.
• We only need the first-octet range associated with “Class B.”
• Loopback (127.x.x.x) is a special case and not part of normal unicast ranges.

Concept / Approach:
In classful addressing, the first octet determines the class: Class A = 1–126, Class B = 128–191, Class C = 192–223, Class D (multicast) = 224–239, and Class E (experimental) = 240–255. The binary prefix for Class B is “10xxxxxx,” which maps to decimal 128–191 inclusive.

Step-by-Step Solution:

Verification / Alternative check:
Converting 128 to binary gives 10000000 and 191 gives 10111111; both begin with 10, confirming Class B. The 127.x.x.x block is reserved (loopback), so it is not a valid Class A host range nor part of Class B.

Why Other Options Are Wrong:

• 128–255: includes Classes C, D, and E.
• 1–127: mainly Class A plus loopback (127).
• 192–223: this is Class C.
• 127–191: starts at loopback (127), which is not a valid network range.

Common Pitfalls:
Mixing up Class B with Class C; forgetting that 127 is reserved for loopback and not counted within Class A usable ranges.

Final Answer:
128–191.