What is the default classful subnet mask on class B networks?

The default classful subnet mask for Class B networks is indeed 255.255.0.0. Class B addresses are defined by their range from 128.0.0.0 to 191.255.255.255, and the default subnet mask allows for a significant number of hosts within a subnet, specifically up to 65,534 hosts, which is appropriate for larger networks that require extensive address allocations. This subnet mask divides the IP address into the network portion (the first two octets) and the host portion (the last two octets), ensuring efficient routing and management of a Class B network.

Other options provided are variations of subnet masks that can be used within a Class B network for subnetting purposes, but they are not the default. For example, the mask 255.255.240.0 allows for more subnets but fewer hosts, while 255.255.255.0 and 255.255.255.224 are commonly used for Class C ranges, where the focus is on smaller networks. Hence, the correct classful subnet mask for Class B remains 255.255.0.0 as the default.