In a Class B address, what is the significance of the second bit?

• A. It must always be turned on.
• B. It must always be turned off.
• C. It indicates a multicast address.
• D. It is reserved for future use.

Answer: (B)

In classful IPv4 addressing, the class of an address is determined by the pattern of the most significant bits in the first octet. For Class B, the first two bits are 10, which means the second bit is always 0. So the significance of the second bit is that it must be turned off. This pattern helps identify the class and, in classic term, allocates 14 bits for the network part and 16 bits for hosts within that network.

For example, a Class B address starts with binary 10 in the first octet (such as 128.x.x.x, which is 10000000 in binary). The second bit here is indeed 0, matching the Class B requirement. While modern networks use CIDR, the concept remains useful for understanding older routing and address allocation schemes.

The second bit does not indicate multicast (that’s Class D, which starts with 1110) and it isn’t reserved for future use. It is simply fixed to 0 for Class B addresses.