When I check the "Validate user IP address" box uder Systems settings>Advanced I get the following notices:

Notice: Undefined offset: 1 in /SugarCRMfolder/index.php on line 248
Notice: Undefined offset: 2 in /SugarCRMfolder/index.php on line 248

That is the following part

// match class C IP addresses
for($i = 0; $i < 3; $i ++) {
248 if($session_parts[$i] == $client_parts[$i]) {
$classCheck = 1;
continue;
} else {
$classCheck = 0;
break;
}
}

I am running Linux-2.6, Apache-2.0.55, PHP-5.0.4, MySQL-4.1.18, SugarCRM-4.2.0d.
SugarCRM is only accessible from the internal network with IP adresses 192.168.1.1-192.168.1.254.