To use a subnet or classless address, table under routers and hosts that have address must keep two pieces of information with every address: the 32-bit address itself and another 32-bit number that specifies the boundary that is called as the Subnet Mask or Address Mask.
Suppose
D = Destination Address
(A, M)= (32-bit IP Address, 32-bit Address Mask)
A = = (D & M)
Now as an example take a 32-bit mask:
11111111 11111111 00000000 00000000
Which can be denoted in dotted decimal number as 255.255.0.0.
Consider a network prefix:
10000000 00010010 00000000 00000000
Which can be shown in dotted decimal number as 128.10.0.0.
Consider a destination address: 128.10.2.3
That has Binary equivalent as:
10000000 00010010 00000010 00000011
A logical 'and' between M and D gives the binary result as:
10000000 00100010 00000000 00000000
Which is similar to prefix 128.10.0.0.