Convert an IP address to an integer in C

This is a C program which converts a string containing an IP address to an integer.

#include <stdio.h>

/* '0.0.0.0' is not a valid IP address, so this uses the value 0 to
   indicate an invalid IP address. */

#define INVALID 0

/* Convert the character string in "ip" into an unsigned integer.

   This assumes that an unsigned integer contains at least 32 bits. */

unsigned int ip_to_int (const char * ip)
{
    /* The return value. */
    unsigned v = 0;
    /* The count of the number of bytes processed. */
    int i;
    /* A pointer to the next digit to process. */
    const char * start;

    start = ip;
    for (i = 0; i < 4; i++) {
        /* The digit being processed. */
        char c;
        /* The value of this byte. */
        int n = 0;
        while (1) {
            c = * start;
            start++;
            if (c >= '0' && c <= '9') {
                n *= 10;
                n += c - '0';
            }
            /* We insist on stopping at "." if we are still parsing
               the first, second, or third numbers. If we have reached
               the end of the numbers, we will allow any character. */
            else if ((i < 3 && c == '.') || i == 3) {
                break;
            }
            else {
                return INVALID;
            }
        }
        if (n >= 256) {
            return INVALID;
        }
        v *= 256;
        v += n;
    }
    return v;
}

/* The following is a test of the above routine. */

int main ()
{
    int i;
    /* "ips" contains a selection of internet address strings. */
    const char * ips[] = {
        "127.0.0.1",
        "110.3.244.53",
        /* The following tests whether we can detect an invalid
           address. */
        "Bonzo Dog Doo-Dah Band",
        "182.118.20.178",
        /* The following tests whether it is OK to end the string with
           a non-NUL byte. */
        "74.125.16.64  ",
        "1234.567.89.122345",
    };
    int n_ips;

    /* Set "n_ips" to the number of elements in "ips". */

    n_ips = sizeof (ips) / sizeof (const char *);

    for (i = 0; i < n_ips; i++) {
        unsigned integer;
        const char * ip;

        ip = ips[i];
        integer = ip_to_int (ip);
        if (integer == INVALID) {
            printf ("'%s' is not a valid IP address.\n", ip);
        }
        else {
            printf ("'%s' is 0x%08x.\n", ip, integer);
        }
    }
    return 0;
}

(download)

The output of the example looks like this:

'127.0.0.1' is 0x7f000001.
'110.3.244.53' is 0x6e03f435.
'Bonzo Dog Doo-Dah Band' is not a valid IP address.
'182.118.20.178' is 0xb67614b2.
'74.125.16.64  ' is 0x4a7d1040.
'1234.567.89.122345' is not a valid IP address.