CODEWITHSUNDEEP
cip-addressbitwise-operators
John Smith
John Smith

Reputation: 85

Converting subnet mask prefix with C

I'd like to convert a prefix like /24 to 255.255.255.0 using bitwise operations.

I have tried using unsigned int like so: 

unsigned int mask = -(1 << 32 - prefix);

I am thinking of creating a while loop that adds 1 to the correct place and then decrements to 0.

All help would be appreciated!

Upvotes: 1

Views: 9588

Answers (2)

Alexey Frunze
Alexey Frunze

Reputation: 62048

Use

unsigned long mask = (0xFFFFFFFF << (32 - prefix)) & 0xFFFFFFFF;

printf("%lu.%lu.%lu.%lu\n", mask >> 24, (mask >> 16) & 0xFF, (mask >> 8) & 0xFF, mask & 0xFF);

Upvotes: 4

Amadeus
Amadeus

Reputation: 10655

Have you tried?

#include <stdint.h>
uint32_t mask = (-1) << (32 - prefix);

once -1 is 0xFFFFFFFF in 2-complement notation, it does the work

Upvotes: 1

Related Questions