Cause of undefined offset notice

[Jennifer <jennifer@xxxxxxxxxxxxxxxxxxxx>]

Hello,

	I have a script that compares a visitors IP address against a file of known bad addresses but I'm getting this notice:

Notice: Undefined offset: 3 (line 26 below)

	Can anyone see what I've done wrong?  Perhaps this sub was not written correctly to begin with?

Thank you,
Jenni

-----------------------------------------------------------

function validate_ip($ip_array, $cm) {
	$msg = '';
	if (isset($cm['bad_ip_message'])) {
		$msg = $cm['bad_ip_message'];
	}
	if (@strpos( $cm['remote_ip'], ':' ) !== false) { return false; }  // Skip IPv6 addresses (?).

	if (!is_array($ip_array)) { return false; }
	foreach ($ip_array as $banned_ip) {
		$banned_ip = trim($banned_ip);
		if ($banned_ip[0] === '#' || $banned_ip === '') { continue; }  // Skip comments and blank lines.
		
		if ( @strpos( $banned_ip, '/' ) === false ) {
			if (ip2long($banned_ip) === ip2long($cm['remote_ip'])) {
				ban_message($msg);
			}
			if (substr($banned_ip, -1) == '0') {
				$banned_ip .= '/24';  // Create a CIDR if IP ends in '0'.
			} 
		} 
		if ( @strpos( $banned_ip, '/' ) !== false ) {
			// Get the base and the bits from the CIDR.
			list($base, $bits) = explode('/', $banned_ip);

			// Now split it up into it's classes.
			list($a, $b, $c, $d) = explode('.', $base);  // Notice: Undefined offset: 3

			// Now do some bit shifting/switching to convert to ints.
			$i    = ($a << 24) + ($b << 16) + ($c << 8) + $d;
			$mask = $bits == 0 ? 0: (~0 << (32 - $bits));

			// Here's our lowest int.
			$low = $i & $mask;

			// Here's our highest int.
			$high = $i | (~$mask & 0xFFFFFFFF);

			// Now split up the IP we're checking against into classes.
			list($a, $b, $c, $d) = explode('.', $cm['remote_ip']);

			// Now convert the IP we're checking against to an int.
			$check = ($a << 24) + ($b << 16) + ($c << 8) + $d;

			// If the IP is within the range, including highest/lowest values,
			// then it's within the CIDR range.
			if ($check >= $low && $check <= $high) {
				ban_message($msg);
			}
		}
	}
	return;
}
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