ipv4_broadcast_addr.c

#include <cstdint>
#include <arpa/inet.h>

/** Returns the IPv4 broadcast address for a given subnet.
 *
 * Assuming host is little endian.
 * Assuming your host is Linux :)
 *
 * subtracting 1 from the return will yield last usable host address.
 *
 * @param prefix : network subnet (network byte order).
 * @param length : CIDR bit length.
 * @return       : highest possible IPv4 address (network byte order).
 */
uint32_t ipv4_broadcast_addr(const uint32_t &prefix, const uint8_t &length) {

    /* A bit length > 32 is invalid.
     * Returning 0 is not safe and may represent
     * a default route in nearly all cases */
    if(length > 32) return 0;

    // fill netmask with 255.255.255.255 and shift left the remaining bits
    uint32_t netmask = 0xFFFFFFFF << (32 - length);

    // temp value, convert network byte order to host byte order
    uint32_t tmp_prefix = ntohl(prefix);

    // add bits from inverse netmask to the subnet
    tmp_prefix |= (~netmask);

    // return network byte order of our IP
    return htonl(tmp_prefix);
}