Saltin,
The subnet mask is a way to "create subnetworks from a network class". The class of an IP address is determined by the most significant bits in the first octet.
0xxx = Class A 1. - 126.
01111111 = reserved 127.
10xxxx = class B 128.-191.
110xxx = class C 192.-223.
111xxx = class D (multicast)
True that in today's classless world we don't talk or use network class much anymore, but for the concept of subnetting to really sink in you have to talk about classfull addressing first. /28 is indeed classless notation, but it doesn't change the fact that the first octect is 192, making that a class C address.
-edit- I know you know the ins and outs of IP addressing, I'm just trying to do a through explanation for other folks.
