c code for valid netmask

Question

I there any way to check that 32 bit netmask is valid or not using bitwise operator?

I have to check from msb side that '1' are in continuous stream or not. eg 11111111.0.0.0 (255.0.0.0)is valid but 11111101.0.0.0 (253.0.0.0) is not.

Answer 1

First thing to do is to check for the netmask being non zero (a nasty edge case). Given this is ok, you need to take the bitwise inverse.

uint32_t y = ~x;

Then add one

uint32_t z = y + 1;

Then if x was a proper netmask, there will be at most 1 bit set in this.

To test that, simply and z with z - 1, which happens to be y. The result will be zero if all is OK, non zero otherwise.

valid = (z & y) == 0;