Please look at the subnetting information again. The last octet isn't supposed to be manipulated in that manner.
Assuming classical routing
1 computer & the router means that you need 2 IP's in addition to the network and broadcast, so you need to unmask the last 2 bits => 11111100 (255.255.255.252)
2 computer & the router (plus network and broadcast) means that you need to unmask the last three bits => 11111000 (255.255.255.248), and this means that you have room in the subnet for one more node (computer, printer, NAS, whatever attaches to the network
4 nodes & the router means that you need to unmask the last 4 bits => 11110000 (255.255.255.240), this allows for up to eight total IP's to be in the subnet.
Normally this should be as far as you would need to go in a regular household. Granted if you have more that 8 nodes and less than 16 the 224 subnet is what you need.
my 2 cents. (too much cisco stuck in my head, I guess that means the Prof did a good job....)

P.S. Of course if you throw out the classical rules and go VLSM or CIDR then all the rules change. you no longer need to reserve two IP's.