In a Class B IPv4 address, how many octets identify the network portion?

• A. One
• B. Two
• C. Three
• D. Four

Answer: (B)

In classful IPv4 addressing, the boundary between the network and host portions is determined by the address class. For Class B, the default subnet mask is 255.255.0.0, so the first two octets identify the network and the remaining two octets identify hosts within that network. That means the network portion consists of two octets (16 bits). For example, in an address like 172.16.45.100, the network is 172.16 and the host is 45.100. If you think about the other extremes, one octet would correspond to the network portion in Class A, and three octets would correspond to the network portion in Class C, but for Class B the network part is two octets.