Some IP utility methods

ip_utils.c

#include <stdio.h>
#include <string.h>
#include <stdint.h>

/**
 * Convert an IPv4 address string into a long
 *
 * @param ip_addr IPv4 address string to convert
 * @return long representation of IP
 */
uint32_t ip2long(char *ip_addr) {
    uint32_t long_addr = 0;
    char *dot = ".";

    char *token = strtok(ip_addr, dot);
    while (token != NULL) {
        long_addr <<= 8;
        long_addr |= atoi(token) & 0xFF;
        token = strtok(NULL, dot);  // get next token
    }

    return long_addr;
}

/**
 * Convert a long into an IPv4 address string 
 *
 * @param long_addr long to convert
 * @param ip_addr_out output buffer for IPv4 address string
 */
char *long2ip(uint32_t long_addr, char *ip_addr_out) {
    uint8_t i;
    char octet[4];
    char *dot = ".";

    ip_addr_out[0] = 0;
    for (i = 0; i < 4; ++i) {
        snprintf((char *)&octet, 4, "%d",
                 (long_addr >> (24 - 8 * i)) & 0xFF);
        strcat(ip_addr_out, (char *)&octet);
        if (i < 3) {
            strcat(ip_addr_out, dot);
        }
    }

    return ip_addr_out;
}

/**
 * Matches IP addresses using net_bits significant bits.
 *
 * @param test_ip_addr IPv4 address to test
 * @param match_ip_addr matching IPv4 address or network
 * @param net_bits number of network bits to use when matching against match_ip_addr (0-32)
 * @return 1 if match or 0 if no match
 */
int ip_matches(char *test_ip_addr, char *match_ip_addr, uint8_t net_bits) {
    uint32_t mask = ~0UL << (32 - net_bits);
    return ((ip2long(test_ip_addr) & mask) == (ip2long(match_ip_addr) & mask)) ? 1 : 0;
}

int main(int argc, char **argv) {
    uint8_t net_bits = 24;
    printf("matches? %d\n", ip_matches("192.168.2.1", "192.168.2.0", net_bits));
    
    return 0;
}