A short example to show how to check and ip address is actually an ip address in C. I have seen a number of time that people provide solution using reg'ex and various other methods to attempt to do this. However there is a standard function that will test for you.

The function is called inet_pton will be able to tell you if a string is acutally an ip address or not. Typically the function also has another use which is to convert an ip address into its network byte ordered integer format.

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

#include <arpa/inet.h>

int main(int argc, char **argv) {
    unsigned long ip = 0;

    if (argc < 2) {
        printf("Usage: %s <ipv4>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (0 == inet_pton(AF_INET, argv[1], &ip)) {
        printf("Failed\n");
        exit(EXIT_FAILURE);
    }

    printf("IP: %ld\n", ip);
    return 0;
}