What do you want to know about Classless Internetwork Domain Routing? Just know how to figure your max nets/subnets/hosts for each amount of network bits. For instance:
4th octet of IP:
10000000 /25 255.255.255.128 (Not suppose to be legit in Cisco exams (also /31))
11000000 /26 255.255.255.192
Now to find out what figures you have:
Let's use 192.168.12.31 as the host, and 255.255.255.192 as the subnet mask.
Ok. Time to figure up the subnets.
256 - 192 = 64
64 = 1st subnet
128 = 2nd subnet
192 = not valid since it's the broadcast addy
Now to figure up the hosts. Subtract 2 from the 64 you got from subtracting 192 from 256. 62 hosts.
First host: 65
Last host: 126
First host: 129
Last host: 190
Hope that clears something up. :P