If the first bit of an IPv4 address is 0, to which class does it belong?

• A. Class A
• B. Class B
• C. Class C
• D. Class D

Answer: (A)

In IPv4’s historical classful addressing, the leading bits of the first octet identify the class. If the very first bit is 0, that means the address belongs to Class A. This pattern corresponds to the default network mask of 255.0.0.0, where the network portion is just the first octet. Practically, usable Class A networks run from 1 to 126 in the first octet (0 and 127 are reserved). The other classes use different starting bit patterns: Class B starts with 10, Class C with 110, and Class D with 1110 (multicast). So a first bit of 0 clearly indicates Class A.