plz i need your help

i  have been asked to design a basic local area network,for a small company,they have 100hosts in the network.you have been asked to provide a detailed plan of the physical layout of this local area network, their will be a single gateway from this network whcih will be administered by isp , peripheral devices must be included,host machines,network devices,severs and peripherals,,,,i must produce potential sources and prices for the devices.i have been given class b ip address 172.16.0.0 and asked to perform subnetting to provide efficient address allocation to all devices in the internetwork which includes the 100 client machines on site, it should be presented in a table formatted.

from my own ideas,subnest mask should be 225.255.128.0 since we borrowed 7bit to create 100 hosts , what the increment  would it be ? would it be 128 ?

I have not checked this well but i think the increment should be 256.0-128.0 =128.0 since this is a class B ip. So you have 128.0, 256.0 etc.