What's the best way to convert from network bitcount to netmask?

Question

For example, if I have a network spec like 172.20.10.0/24, "24" is the bitcount. What's the best way to convert that to a netmask like 0xffffff00 ?

Answer 1

Why waste time with subtraction or ternary statements?

int suffix = 24;
int mask = 0xffffffff ^ 0xffffffff >> suffix;

If you know your integer is exactly 32 bits long then you only need to type 0xffffffff once.

int32_t mask = ~(0xffffffff >> suffix);

Both compile to the exact same assembly code.