In a course that I will be teaching in Boston, Management 512, Security Leadership Essentials for Managers, there is a lab that asks how to cut a network in half.
NOTE: If you are a current or prospective MGT 512 student, do not panic. You are not expected to know the fine points below. The context of the lab is that you are conducting a job interview and the candidate is a network engineer that is expected to know this material.
Original network: 172.16.18.0/24
- Public or private? Private
- Network and broadcast addresses: 172.16.18.0/24, broadcast 172.16.18.255
- Range of valid host addresses: 172.16.18.1 - 172.16.18.254 with exceptions:
The .0 address is the network address and is not a usable host address. When we change the subnet mask to 255.255.255.128 or /25, the two "network" addresses for the same range become 172.16.18.0 and 172.16.18.128. The broadcast addresses become 172.16.18.127 and 172.16.18.255. There are now four addresses that are not usable: .0, .127, .128, .255.
Cut the network in half. 172.16.18.0/25
172.16.18.0 = network address 1
172.16.18.127 = broadcast address 1
172.16.18.1 to 172.16.18.126 valid host addresses
172.16.18.128 = network address 2
172.16.18.255 = broadcast address 2
172.16.18.129 to 172.16.18.254 valid host addresses