inet_pton

#define    IN6ADDRSZ    16
#define    INADDRSZ     4
#define    INT16SZ         2

#ifndef    AF_INET6
#define    AF_INET6    AF_MAX+1    /* just to let this compile */
#endif

/*
* WARNING: Don't even consider trying to compile this on a system where
* sizeof(int) < 4.  sizeof(int) > 4 is fine; all the world's not a VAX.
*/

static int    inet_pton4(const char *src, u_char *dst);
static int    inet_pton6(const char *src, u_char *dst);

/* int
* inet_pton(af, src, dst)
*    convert from presentation format (which usually means ASCII printable)
*    to network format (which is usually some kind of binary format).
* return:
*    1 if the address was valid for the specified address family
*    0 if the address wasn't valid (`dst' is untouched in this case)
*    -1 if some other error occurred (`dst' is untouched in this case, too)
* author:
*    Paul Vixie, 1996.
*/
int inet_pton(int af, const char *src, void *dst)
{
    switch (af)
    {
        case AF_INET:
            return (inet_pton4(src, dst));
        case AF_INET6:
            return (inet_pton6(src, dst));
        default:
            errno = EAFNOSUPPORT;
            return (-1);
    }
    /* NOTREACHED */
}

/* int
* inet_pton4(src, dst)
*    like inet_aton() but without all the hexadecimal and shorthand.
* return:
*    1 if `src' is a valid dotted quad, else 0.
* notice:
*    does not touch `dst' unless it's returning 1.
* author:
*    Paul Vixie, 1996.
*/
static int inet_pton4(const char *src, u_char *dst)
{
    static const char digits[] = "0123456789";
    int saw_digit, octets, ch;
    u_char tmp[INADDRSZ], *tp;

    saw_digit = 0;
    octets = 0;
    *(tp = tmp) = 0;
    while ((ch = *src++) != '\0')
    {
        const char *pch;

        if ((pch = strchr(digits, ch)) != NULL)
        {
            u_int new = *tp * 10 + (pch - digits);

            if (new > 255)
                return (0);
            *tp = new;
            if (! saw_digit)
            {
                if (++octets > 4)
                    return (0);
                saw_digit = 1;
            }
        }
        else if (ch == '.' && saw_digit)
        {
            if (octets == 4)
                return (0);
            *++tp = 0;
            saw_digit = 0;
        }
        else
            return (0);
    }
    if (octets < 4)
        return (0);

    /* bcopy(tmp, dst, INADDRSZ); */
    memcpy(dst, tmp, INADDRSZ);
    return (1);
}

/* int
* inet_pton6(src, dst)
*    convert presentation level address to network order binary form.
* return:
*    1 if `src' is a valid [RFC1884 2.2] address, else 0.
* notice:
*    (1) does not touch `dst' unless it's returning 1.
*    (2) :: in a full address is silently ignored.
* credit:
*    inspired by Mark Andrews.
* author:
*    Paul Vixie, 1996.
*/
static int inet_pton6(const char *src, u_char *dst)
{
    static const char xdigits_l[] = "0123456789abcdef",
        xdigits_u[] = "0123456789ABCDEF";   
    u_char tmp[IN6ADDRSZ], *tp, *endp, *colonp;
    const char *xdigits, *curtok;
    int ch, saw_xdigit;
    u_int val;

    memset((tp = tmp), 0, IN6ADDRSZ);
    endp = tp + IN6ADDRSZ;
    colonp = NULL;

    /* Leading :: requires some special handling. */
    if (*src == ':')
        if (*++src != ':')
            return (0);
    curtok = src;
    saw_xdigit = 0;
    val = 0;
    while ((ch = *src++) != '\0')
    {
        const char *pch;

        if ((pch = strchr((xdigits = xdigits_l), ch)) == NULL)
            pch = strchr((xdigits = xdigits_u), ch);
        if (pch != NULL)
        {
            val <<= 4;
            val |= (pch - xdigits);
            if (val > 0xffff)
                return (0);
            saw_xdigit = 1;
            continue;
        }
        if (ch == ':')
        {
            curtok = src;
            if (!saw_xdigit)
            {
                if (colonp)
                    return (0);
                colonp = tp;
                continue;
            }
            if (tp + INT16SZ > endp)
                return (0);
            *tp++ = (u_char) (val >> 8) & 0xff;
            *tp++ = (u_char) val & 0xff;
            saw_xdigit = 0;
            val = 0;
            continue;
        }
        if (ch == '.' && ((tp + INADDRSZ) <= endp) && inet_pton4(curtok, tp) > 0)
        {
            tp += INADDRSZ;
            saw_xdigit = 0;
            break;    /* '\0' was seen by inet_pton4(). */
        }
        return (0);
    }
    if (saw_xdigit)
    {
        if (tp + INT16SZ > endp)
            return (0);
        *tp++ = (u_char) (val >> 8) & 0xff;
        *tp++ = (u_char) val & 0xff;
    }
    if (colonp != NULL)
    {
       /*
            * Since some memmove()'s erroneously fail to handle
            * overlapping regions, we'll do the shift by hand.
            */
        const int n = tp - colonp;
        int i;

        for (i = 1; i <= n; i++)
        {
            endp[- i] = colonp[n - i];
            colonp[n - i] = 0;
        }
        tp = endp;
    }
    if (tp != endp)
        return (0);
    /* bcopy(tmp, dst, IN6ADDRSZ); */
    memcpy(dst, tmp, IN6ADDRSZ);
    return (1);
}

This may be your first encounter with the inet_pton function. It is new with IPv6.

Older code uses the inet_addr function to convert an ASCII dotted-decimal string into the correct format, but this function has numerous limitations that inet_pton corrects.