subnetmask bit 정리(계산하기 귀찮으니)

subnet(netmask) / bit  / host
255.255.255.0    / 24   / 254
255.255.255.128 / 25   / 128
255.255.255.192 / 26   / 64
255.255.255.224 / 27   / 32
255.255.255.240 / 28   / 16
255.255.255.248 / 29   / 8
255.255.255.252 / 30   / 4
255.255.255.254 / 31   / 2
255.255.255.255 / 32   / 1