This is good for Cisco exams, and work, if you ever need to figure out subnets on paper.
To figure out the valid addresses in a subnet:
Take the octet that is not 0 or 255 and subtract that value from 256 - i.e. 256 - 224 = 32. Now make a chart starting at 0, and adding the value from that last step with each line (the note board I got for the CCNA had gridlines, which made it even easier)
0
32
64
96
128
160
192
224
256
Now, leaving room in between, write on each line the value of the line below, minus 1
0 31
32 63
64 95
96 127
128 159
160 191
192 223
224 255
256
The values on the left are the valid subnet addresses, and the right is the broadcast address.
If you want, now fill in the valid ranges in between
0 1-30 31
32 33-62 63
64 63-94 95
96 97-126 127
128 127-158 159
160 161-190 191
192 192-222 223
224 225-254 255
256
There you go, all your addresses, with subnet and broadcast addresses.
(Sorry for the formatting)
