You are setting up a network in which you need 15 subnetworks. You have been given a network address of 153.20.0.0, and you need to maximize the number of hosts in each network. Which subnet mask will you use?

The subnet mask will be 255.255.240.0 (/20). Since you need to solve for the number of networks, the equation is as follows: 2^X is equal to or greater than 15 networks. 2^4 = 16 completed the equation; the 4 bits represent the subnet side; you add the 4 bits to the 16 bits of the class B subnet mandated by the IETF. 16 + 4 = /20 = 255.255.240.0.