In a Class A address, what is the significance of the first bit?

• A. It must always be 0, indicating the address range is between 0 and 127
• B. It must always be 1
• C. It determines the network mask
• D. It has no fixed value

Answer: (A)

The main idea is how classful IPv4 addressing uses the first bit of the first octet to indicate the address class. For Class A, that leading bit is 0. This single bit determines that the network part uses only the first octet (with the default subnet mask 255.0.0.0), so Class A addresses span from 0.0.0.0 to 127.255.255.255 (though 0.x and 127.x are reserved for special purposes in practice). So the first bit being 0 is what designates Class A and thus defines the 0–127 range.

The other statements don’t fit: the first bit is not always 1, it doesn’t by itself set the subnet mask (the class dictates the default mask, though subnetting can change it), and it does have a fixed value for Class A addresses (0), not a completely variable one.