In a /16 subnet, how many bits are used for the network portion?

• A. 16 bits
• B. 8 bits
• C. 24 bits
• D. 32 bits

Answer: (A)

Understanding CIDR notation helps: the number after the slash shows how many leading bits in the 32-bit IPv4 address form the network portion. A /16 means the first 16 bits are used for the network, leaving the remaining 16 bits for hosts. Since IPv4 addresses are 32 bits, 32 minus 16 equals 16 host bits, so the network portion is 16 bits. For example, a /16 like 192.168.0.0/16 uses the first two octets (192 and 168) to define the network, while the last two octets can vary from 0.0 to 255.255, giving 65,536 possible addresses in that network (65,534 usable hosts). The other options would imply different splits: 8 bits for the network would be a /8 with 24 host bits; 24 bits for the network would be a /24 with 8 host bits; 32 bits would leave no host bits at all, representing a single address.