How to cut a network in half

/* This fluff headed manager would like to thank Judy Novak, Andrew Laman and David Hoelzer for their help in creating this blog post */

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