Any valid IP address consists of four numbers separated by three period (.) characters and each nunber ranges from 0-255 inclusive. An example IP address is “125.231.167.234″. The string function sscanf can be used to extract the four numbers and then validate each number for the range 0-255.

The following function returns 0 if the IP address is invalid and 1 if valid.

int is_valid_ip(const char *ip_str)
{
        unsigned int n1,n2,n3,n4;

        if(sscanf(ip_str,"%u.%u.%u.%u", &n1, &n2, &n3, &n4) != 4) return 0;

        if((n1 <= 255) && (n2 <= 255) && (n3 <= 255) && (n4 <= 255)) return 1;
        return 0;
}

Download: validate-ip.c

The output will be as follows if the code in validate-ip.c is executed.

[neo@techpulp ~]# gcc validate-ip.c -Wall -o validate-ip
[neo@techpulp ~]# ./validate-ip

------------------------------------------
IP Address Validation
------------------------------------------
             1.1.1.1    VALID
     255.255.255.255    VALID
           1.255.4.5    VALID
        127.8.190.29    VALID
3hu23e832j2....wjdnw    INVALID
         3a.3b.2d.5t    INVALID
         -1.-1.-1.-1    INVALID
------------------------------------------

[neo@techpulp ~]#