Under the historical classful IPv4 scheme, what is the valid first-octet range for a Class A unicast address (excluding the reserved 0 and the loopback block)?

Introduction / Context: While modern networks use CIDR, many exam questions still reference classful addressing. Class A networks historically had a /8 default mask and a distinct first-octet range. Knowing the range also clarifies special-use blocks such as loopback.

Given Data / Assumptions:

• Class A default mask: /8.
• First octet defines the class in classful addressing.
• Loopback (127.0.0.0/8) is excluded from ordinary unicast.

Concept / Approach: Class A first-octet range is 0–127 in binary terms (leading bit 0). However, the 0.0.0.0 block is reserved (this host/network) and 127.0.0.0/8 is loopback. Therefore, valid assignable Class A unicast addresses have first octet 1 through 126 inclusive.

Step-by-Step Solution: Recall classful ranges: A = 0–127, B = 128–191, C = 192–223.Remove reserved: 0 (special), 127 (loopback).Remaining valid unicast first octet for Class A is 1–126.

Verification / Alternative check: The dotted-decimal blocks beginning with 10.x.y.z (private Class A) sit within this 1–126 range; operating systems treat 127.x.y.z as loopback, reinforcing the exclusion.

Why Other Options Are Wrong: 1–127: Includes 127 (loopback).

128–191: That is Class B.

192–223 and 192–255: Those ranges overlap Classes C and beyond; not Class A.

Common Pitfalls: Forgetting the special-case 127/8 loopback block and accidentally including it in the Class A usable range.

Final Answer: 1 - 126