Number of hosts in a subnet

Question

Does anyone know how many host bits are needed to guarantee that a subnet could have 9 usable hosts?

I was thinking about around 4, but I'm not sure. Can some shed some light on this?

Answer 1

Two subnet addresses (all-ones and all-zeros) can't be used to indicate a host, so with N bits you get up to (2**N - 2) usable hosts. So, for 9 hosts, 4 bits is correct: it would do up to 14, but 3 bits would do only 6 hosts.

Answer 2

Here are networks that meet the requirement 192.168.1.0 /28

Mask:255.255.255.240 Host/Net - 14
Network          Broadcast
192.168.1.0      192.168.1.15
192.168.1.16     192.168.1.31
192.168.1.32     192.168.1.47
192.168.1.48     192.168.1.63
192.168.1.64     192.168.1.79
192.168.1.80     192.168.1.95
192.168.1.96     192.168.1.111
192.168.1.112    192.168.1.127
192.168.1.128    192.168.1.143
192.168.1.144    192.168.1.159
192.168.1.160    192.168.1.175
192.168.1.176    192.168.1.191
192.168.1.192    192.168.1.207
192.168.1.208    192.168.1.223
192.168.1.224    192.168.1.239
192.168.1.240    192.168.1.255