#include <arpa/inet.h>   // for inet_pton

static int is_ipv4_prefix(const char *host, size_t *prefix_len)
{
    size_t i = 0;
    while (host[i] && ((host[i] >= '0' && host[i] <= '9') || host[i] == '.'))
        i++;
    if (i == 0)
        return 0;
    char buf[32];
    if (i >= sizeof(buf)) // too long to be valid IPv4
        return 0;
    memcpy(buf, host, i);
    buf[i] = '\0';
    struct in_addr addr;
    if (inet_pton(AF_INET, buf, &addr) == 1) {
        *prefix_len = i;
        return 1;
    }
    return 0;
}