The manual states that "ip2long() will also work with non-complete IP addresses", however this is system-dependant so cannot be relied upon. For example, on my system ip2long() will return FALSE for incomplete addresses:
<?php
var_dump(ip2long("255.255")); // bool(false)
?>
This is because ip2long will use inet_pton if available, which does not support non-complete addresses. If inet_pton is not available on your system, inet_addr will be used and incomplete addresses will work as stated.