TYPO3 Core

first:
[BE][lockIP] is per default 4. this means in usersession the ip is saved in 4 parts
aaa.bbb.ccc.ddd
you can reduce this info by [BE][lockIP] , but valid is only 1-4

Your setting 0 will be changed to 1 and saves only aaa

Ok, the error- here this only works with 4 parts, so it has to be fixed like

$hostByAddr = @gethostbyaddr($session['ses_iplock']) ? gethostbyaddr($session['ses_iplock']) : '';
$hostName = ($session['ses_iplock'] != '[DISABLED]' ? $hostByAddr : '[DISABLED]');

i've changed minimal, as if gethostbyaddr fails, i show now $session['ses_iplock'] instead of empty (use other section if want to hide)