binary representation of ip prefixes
For a netmasked IP address xxx.xxx.xxx.xxx/N, convert each xxx octet in the address to binary.  The masked address is the leftmost N bits with all other bits replaced with zeroes.

So 10.0.0.2/2 becomes 00001010 00000000 00000000 00000010 which is then masked to 00000000 00000000 00000000 00000000, because 00 are the leftmost 2 bits.

90.0.0.0/4 becomes 01011010 00000000 00000000 00000000 which is then masked to 01010000 00000000 00000000 00000000, because 0101 are the leftmost 4 bits.