Correct and valid mask

The correct and valid mask must meet certain conditions. After the decimal mask is converted into binary, the left part of the mask must be 1 and there must be no 0 in the middle. For example, if you convert 255.255.248.0 to a binary value of 11111111.111111.11111000.00000000, you can see that the values on the left are 1 and there is no 0 in the middle of 1 (0 is on the right of 1). This is a valid mask. Let's take a look at 254.20.248.0 and convert it to a binary value of 11111110.111111.11111000.00000000. This is not a correct and valid mask because there is a 0 in the middle of 1.