How to check if an ip address belongs to a specific subnet?

Question

Computer networks

Greetings, I didn’t find a clear explanation on the network on how to determine whether the IP address belongs to a specific subnet, how to find the network address, broadcast or match the logical "AND" IP and the mask, I know! Here is an example: which subnet belongs to ipi 10.120.100.255/18 address options
1) 10.120.0.100
2) 10.0.0.0
3) 10.120.64.0
4) 10.120.96.0
PS I do not need a solution, I need the correct calculation algorithm.

Answer 1

10.120.100.255/18
First represent the address and mask in binary form
00001010.01111000.01100100.11111111/11111111.11111111.11000000.00000000
Then bitwise AND the mask and address.
00001010.01111000.01000000.00000000
Then convert back to decimal.
10.120.64.0

Answer 2

You check the left side of the bits and that's it.
And the physical one and the same IP can be on different continents, different subnets (a popular example is 127.0.0.1)