You need 500 subnets, each with about 100 usable host address per subnet. What…

Answer: 255.255.255.128 (/25)

• Start with a class B network: default mask is /16, so there are 16 host bits available to subnet and host addressing.

• You need at least 500 subnets: calculate required subnet bits = ceil(log2(500)) = 9, because 2^9 = 512 ≥ 500.

• Host bits left = 16 - 9 = 7, so usable hosts per subnet = 2^7 - 2 = 126, which satisfies the ≈100 host requirement.

• Final mask = / (16 + 9) = /25, which in dotted decimal is 255.255.255.128.

Therefore the correct network mask to assign is 255.255.255.128 (/25).